Hi, here's a guide for Sona, work in progress (no fancy graphics here). Decided to make a guide for a friend, first time making a guide, so criticism is welcome (especially from Sona mainers; I don't play her much so i don't know any specific strategies)!

Pros:
+Very good sustain in lane with her Aria of Perseverance
+Poking game in lane is strong and consistent with Hymn of Valor
+Great utility; has more tools to help others than most supports
+ Crescendo can swing team fights hugely in your favor
+Sick beats
+10/10 butt

Cons:
-Weak against aggressive lanes that can kill your carry quickly( Blitzcrank, Thresh, Leona etc.)
-Difficult to setup kills prior to Level 6 and Crescendo, leading to a more passive lane
-Has to be close to the action to get the most out of her auras; can lead to death if not mindful

We pick up Exhaust to protect the AD carry and help to setup kills.

We pick up Flash to protect ourselves and potentially get some sweet Crescendo action.

Pretty simple.

Runes are pretty subjective. Everyone has their own preferences, amounts, some people even mix and match their runes, putting 4 of this 5 of that, etc...

(Could use some feedback here; don't know much about runes)

The runes I have listed are general defense runes; they work with every champion, so it's a safe bet to get these first on hitting level 30.

Seals

Greater Seal of Armor is generally the only seal you'll use. Greater Seal of Mana Regeneration is also worth considering, but having the extra defense will help more for the most part, especially in the laning phase.

Marks

Greater Mark of Armor is not the most optimal rune. You can see that seals give more armor than marks. Marks are generally a slot for the more offensive runes. However, we are playing a support here, and so it's fine if you want to give up more damage for defense. The best offensive options are Greater Mark of Magic Penetration and Greater Mark of Hybrid Penetration. Magic penetration runes are good for obvious reasons. Hybrid penetration is specifically decent on Sona because of her passive, Power Chord; hybrid runes would give her passive a little more oomph when you attack someone with it. You can also technically use these runes on more champions, but it'll be suboptimal compared to using just armor pen/magic pen purely. Magic penetration runes are a safer bet, for sure.

Glyphs

Your glyph choices might need a bit more thought. I use Greater Glyph of Scaling Magic Resist over Greater Glyph of Magic Resist for defensive purposes. Both are equally viable. The reason why I believe scaling runes are better here is because while you are laning, you will not be dealing with heavy magic damage. You will be poked here and there by a spell, but for the most part, spells will not be what kills you; physical attacks will be. That's why it's important to use straight armor runes; to give you an early game edge It's important to get plain magic resist runes middle because it can make the difference between winning or losing, but as a support with a heal, it's just not as important. However, the decision is ultimately up to you; do you want more defense early on, or more later?

Another important glyph to consider is Greater Glyph of Cooldown Reduction. This glyph just gives you faster spells throughout the game, which will increase your throughput of heals, damage, etc. They're not a bad option; it may be hard to see the effect, but over the course of a game it may make the difference. Of course, there's also the purely offensive option in Greater Glyph of Ability Power, but I wouldn't recommend it.

Quints

As far as quints go, if you want to go the defensive route, I would recommend just picking up Greater Quintessence of Health. They just work in every situation regardless of the opponents setup, and they work perfectly fine on any ol' champion. The rest of the quints give you pretty much every stat in the game, so if you want to look around and see what you'd like the most, then feel free to. If you feel like you're mana starved, go for the Greater Quintessence of Mana Regeneration. If you want more cooldown reduction, go for it. If you want to be an ***hat, go for Greater Quintessence of Revival. The choice is yours. That applies to all the other runes of course; you can mess around! You've got to have a goal to start with, though.

Time to break down items!

Starting Items

Spellthief's Edge

Out of all the gold income items, this is your best option. Gives you AP and mana regen, which is exactly what you need. Make sure to keep poking the enemy so that you're at least below full stacks of the gold buff it gives at all times.

Ancient Coin

This would be the other option you might want as a starting item. It isn't as commonly used on Sona, as the AP on Spellthief's Edge helps out in lane more. However, it builds into Talisman of Ascension, an interesting item we'll discuss later on. Try both out, and see which you prefer.

Health Potion

Get this (or Total Biscuit of Rejuvenation, obviously) for sustain in lane. Make sure you get full use out of the healing when you use it.

Mana Potion

If you feel that you like to have more mana early game without going back, then there's no shame in mixing these in with your starting items. They are less useful when you have Total Biscuit of Rejuvenation, since biscuits give you some mana as well.

Stealth Ward

It's good to start the game with a ward. You likely won't go back before the enemy jungler is able to gank (especially when against quick gankers like Lee Sin), so having one ward early on the cover your lane is very helpful.

Warding Totem

You start with this over Sweeping Lens because you just want vision of what your opponent is doing before the game starts with the extra ward, without having to waste your actual Sight Ward.

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First Back (Early Game Core)

Sightstone

If you can afford the 800 gold to buy this; always, always, always purchase it first thing you can. It will save you from ganks, it will save other people from ganks, it will allow you to gank their jungler, gank anyone roaming around; it will make your teams pushes that much safer, it will keep track of objectives such as the Dragon; Baron Nashor; blue buff; red buff; the list goes on. Buy it. (mini ward guide to follow)

Stealth Ward

If you can't afford Sightstone make sure to buy 1 or 2 of these, since you need the wards. Don't buy them anymore after you get your sightstone.

Frostfang

This should be the first item you'll want to buy after Stealth Ward. Just upgrades some of your stats going through the laning phase. I would still prioritize buying Stealth Ward though; unless you have 600-700 gold to afford both wards and Frostfang, I would wait until later to purchase, and instead get:

Boots of Speed

Boots. You need them. They help you escape, they help you chase. You might even want to prioritize boots over getting Frostfang if you are against slow projectile opponents that will wreck your face so you can dodge easier ( Nidalee, Blitzcrank, Thresh.

Potions

Consider buying these if you feel like you need them. If you have 35 extra gold after buying other items, definitely go ahead and purchase one.

Vision Ward

Don't buy this on your first back, unless you're rolling in cash. This is just a reminder that you need to get in the habit of keeping your Vision Ward up on the map at all times. If it goes down, buy another one. More on that in the ward guide.

After you get all your items, don't worry about buying it anymore. Having a useful item will be better than keeping an inventory slot free for no reason.

Sweeping Lens

Helps to counterward the opponent. The support gets it since it's redundant to keep Stealth Ward while you have Sightstone (junglers can get it too, since they buy wards often). Use it in key spots where you'll know there might be a ward, especially when doing an objective like Dragon or Baron Nashor.

Basically, if you're in a situation where you don't want to be seen, use Sweeping Lens to, well, not be seen. If you got the ward, then you have that information and can avoid dying thanks to it. Keep in mind that it also disables the ward for the duration, so you can be sneaky if you use it correctly.

Buy Oracle's Lens when you have the spare money later. It's not too crucial, but it's nice to have.

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Leaving Early Game: What to buy next?

At this point, some things will have happened in the game. Maybe their AP carry middle got fed. Maybe you've gotten a lot of kills. Maybe you're dying a lot. Your item choices will depend on what has happened in the game so far, and how best to prepare for the future to come.

Offensive Items

If you are winning, not dying very much, or the game is pretty even, getting an offensive item is a good idea. It can help you to push your team's advantage, as having more offensive power when your enemy is underleveled will help to keep them that way. Not to mention that these items will also assist your healing capabilities.

Frost Queen's Claim

You should upgrade your Frostfang before you consider any other offensive item. It's a cheap way to get stats. The active isn't too amazing, but it can help out in a pinch when you need that slow for escaping or chasing. After getting this item, I would recommend getting a defensive item before picking up another offensive one, unless your team is doing crazy good. It's good to have a balance starting out, so you don't overcompensate in one direction and find yourself too defensive or too offensive as the game expands. Replace this item once your inventory is full. You can get better stats, and you won't need the gold anymore.

Morellonomicon

This is a good pick-up if you're dealing with a champion with heavy healing capabilities ( Soraka, Dr. Mundo, Warwick). It can help to shut down the enemy team's survivability.

Athene's Unholy Grail

A great item for mana regeneration. If you're away from base a lot (pushing heavily), the mana this item provides can allow you to keep your team alive for long periods of time. It also has the bonus of being good against AP champions due to its magic resistance. This is the most conservative offensive item you can pick up, as the stats lend themselves well to the support role. It's a safe bet if you're uncomfortable with having a full on offensive item.

Archangel'z Staff

This item will give you an absolute truckload of mana. In that way, it's similar to Athene's Unholy Grail. It's up to you whether you prefer more mana regen, or more mana in general. Has the added benefit of increasing your survival once you get the mana shield activation of Seraph's Embrace in the late game, which makes it scale better than Athene's Unholy Grail. If you're against heavy AD, you wouldn't need the magic resistance from the grail, making this a better option in those situations (I would think). I would only pick up one of either the grail or archangel's staff in any given game. You don't want to go overboard on the mana.

Lich Bane

This is a cool option that Sona has access to. Power Chord allows her to put out some pretty mean auto-attacks right after spell casts, and Lich Bane will just make those even stronger. Not to mention that her spells have very quick cooldowns, so you can have stronger auto attacks most of the time. A good item to top off any build, if you want some offense in the later game.

Rylai's Crystal Scepter

Only get this if your team is lacking crowd control something serious. The health is nice, but you're mainly buying this for the slow as a support. I don't even think you get the full slow as Sona since she targets multiple enemies. So, only get this item if your team desperately needs it (like a team with no slows or stuns against Udyr or something (but you probably wouldn't catch him anyway(sounds like you're gonna have a hell of a time in that game anyway)))

Zhonya's Hourglass

Another item you should almost never get. As a support, you should be the last one targeted in team fights. But, if for some reason you can't even do anything and find yourself dying constantly, then this would help you survive long enough to get the other team killed/keep yourself alive. If they're targeting you at the beginning of fights they're probably losing anyway though.

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Defensive Items

If you're losing, dying terribly, despairing, etc., you may want to skip the offensive item entirely in the beginning and just get a defensive one first. If you find you're not dying anymore, go ahead and pick up an offensive item. All of these defensive items are reactionary depending on what's happening in the game, so keep the enemy team composition and flow of the game in mind when making a decision.

Locket of the Iron Solari

This item is a great default. It gives you some good defensive stats, gives your team some nice magic resistance (the health regen is kind of irrelevant), and it gives your team a nice shield (230 damage per person = 1150 effective healing in a team fight at level 18! wowie!). The shield especially good against teams with heavy AoE, such as Amumu, Ziggs, Fiddlesticks, etc. A great pickup. However, this is likely the item you'll want to replace with the next item, given the situation:

Mikael's Crucible

This item is meant to save your AD carry. That's what it does. If you feel like the enemy team has enough CC and single target damage to kill your carry before you can even do anything (or if that's already happened), get this item to save them. It's crucial to keep your AD carry alive, and that's your job.

Banshee's Veil

Only get this if you are being absolutely wrecked by their AP carry, or they generally have a lot of AP. Prioritize a team defensive item like the previous two before considering picking this up.

Randuin's Omen

I love this item. It's simple, but works so well. Armor, health, anti AD carry measures, and a slow to boot, which will help you survive or pick up a kill. You have to be in melee range for it to work, though, so don't go out of your way to get the slow off if it's risky. Get this in most games, unless the enemy team is devoid of attack damage for some reason. Armor helps a lot.

Frozen Heart

Only get this if you're freaking out over how fed the enemy AD carry is and no one else is buying it. Randuin's Omen is a better option in most situations, and your tank should be getting Frozen Heart if they're decent enough and see the AD carry is fed (or they might just buy it anyway, since it's a good item).

Ruby Sightstone

Just pick it up eventually. It's not that great, but you already have Sightstone, so you might as well upgrade it. Don't prioritize it.

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Boots

Those things that make you go fast. You'll want to upgrade them eventually, but don't do it if you can get a full item. Just get them whenever you can spare the gold and can't buy something better while you're in the fountain.

Boots of Mobility

Your default boots. They give you extra move speed to ward, arrive at team fights, help various teammates, etc. Being places faster is important for supports.

Ninja Tabi

Get these if you're scared of their AD potential. Gives some nice armor, and a reduction from auto attacks.

Mercury's Treads

Get these if you're scared of their AP potential. Also useful if they have a lot of CC in general due to Tenacity. Probably the defensive boots you'll purchase most frequently.

Ionian Boots of Lucidity

A more aggressive option of boots. Helps your damage, helps your healing, helps it all. You might prefer these over Mobility Boots as a default option. It's up to your playstyle.

Enchantments

Just a quick section on this. Buy your enchantments at some point, usually way later in the game. Enchantment: Homeguard is for when you're being pushed, and you need to get out of base faster. Enchantment: Alacrity is for when you're the one doing the pushing, and the extra movement speed will help in fights and positioning. It's that simple.

You may also want to consider Enchantment: Captain, but since your position is crucial as Sona, I'd say Enchantment: Alacrity is better for the most part.

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Odd items that you may want to try

These items won't be used most games. You might not use them in any games. But, they're there, and deserve to be discussed.

Talisman of Ascension

Talisman of Ascension

This item builds from Ancient Coin. It has a better active than Frost Queen's Claim, which can net you kills and enable escapes for your entire team. However, the stats are fairly mediocre, so it's a debatable pickup.

Twin Shadows

An item with a weird active. It can be used to chase down enemy champions, or can be used to protect your team against incoming enemies while you're pushing. However, it has a super long cooldown if it fails to locate anyone, so it's eh. I haven't tried this item yet, so give it a shot and see what it's like. Just keep in mind that there's safer options.

Ardent Censer

This item is controversial. It seems like an awesome idea, but it isn't effective at all if your AD carry isn't around, you're playing Sona, so you can't even tell if your heals will hit your carry (although you can shield them), and the stats for an item slot are pretty bad. Eh. Wouldn't recommend it.

(Work in progress, just wanted to get item section out for friend)