Hello Everyone and Welcome to my First Guide! I don't know much about coding so sorry if this looks bland right now.
This guide is for the average support Sona. The reason why I chose Sona was because she was one of the first supports I ever tried and she is very easy to play.
But when playing support, always remember to NEVER take your ADC's minions. They need it more than you. The only time I ever take minions is when my ADC is not in lane anymore.

Pros

• Good poke (Q and Passive)
• Good Sustain (Heal)
• Good in team fights with auras
• Stun
• Slow (E and Passive)
• Pretty short cooldowns

Cons

• Squishy
• Low health pool
• Abilities You Can't Aim (Except your Ult)
• Heal isn't very strong for major life saving(versus Soraka Heal)
• Mana hungry if abilities are not used right

With Sona, I max my Q first while dropping points in my W and E early on. After maxing Q, I max my W and drop one or two points in my E as it is INSANELY slow. Your movement speed boost won't really matter if the enemy has high cc or high movement speed.

I chose Greater Quintessence of Gold because it allows Sona to buy more items both early and late game. With the new support items, you'll be getting tons of gold.
For marks, I got Greater Mark of Magic Penetration because it will allow your Q and passive to do more damage later on. You can switch out these marks for anything else, like if you want Greater Mark of Mana. It's all about personal preference.
For seals, I got Greater Seal of Armor because it adds a stronger defense to Sona since she is very squishy.
For glyphs, I choose either Greater Glyph of Ability Power or Greater Glyph of Magic Resist. The choice is up to you whether you want more damage out of your spells or if you want the extra defense to make you that much harder to kill.
Also, you can choose to use scaling runes instead of flat runes. For those who do not know, scaling runes give more boosts per level so there is a greater boosts at level 18 whereas flat runes give flat boosts. Even though the difference is not too great, the extra boost could be vital in killing or surviving.

For summoner spells, I normally take and

will be one of Sona's few escapes. As said before, her E doesn't boost you enough to really matter. Also, you can also flash in the way of skill shots too block off your allies (ex: Caitlyn Ult). You dying is better than your ADC dying.

is a good spell if you want your ADC to catch up to someone. It is also a defensive ability if the enemy's high attack speed ADC is attacking you as it decreases their attack speed. (ex: Varus)

Other good spells are , , , and .
Heal is a good spell for supports in general and is good if your built in heal isn't enough to help out.
Clarity is good if you and your ADC are very mana hungry.
Cleanse is good if you are in a sticky situation with high CC.
Teleport is a good spell if you need to get back in lane quickly after recalling. This spell is also good when paired with Homeguard boost.
The rest of the available spells are not really recommended.
: Just a bad spell for everyone besides the jungler.
: This spell is ok when paired with Teleport but you wouldn't need this if you were being a good support and not dying (except for when you can protect the ADC)
: You can get almost the same effect with items like Talisman of the Ascension for the movement speed boost.
: This ability is ok but you wouldn't need the shield if you don't get range of the enemy. You can also get this effect with items like Locket of the Iron Solari which also shields allies.
: This spell is more for the aggressive roles and it is often used to secure kills. As a support, you should let other people take the kill because they need the gold more. But if no team mate around (which there always should be), feel free to take the kill.
: This ability is just used to scout out locations but everyone should be warding around the map so you don't really need this. The Scrying Orb trinket is basically the same but with shorter range.

Correctly utilizing Sona's passive is very important. Not only does it amplify some of your basic attacks. After every three abilities, Sona's next basic attack is augmented based on the last ability used. They all deal extra magic damage. They also have an added bonus effect but I'll get into those with each individual ability. This is Sona's q, it deals magic damage to two enemies and it prioritizes champions. A good thing with most of Sona's abilities is that you don't have to aim them. They prioritize the most important thing. With this ability, Sona gain's an aura that increases attack damage and ability power. It can be defined by the blue "waves" being emitted by Sona. Sona's Power Chord portion of the ability deals double the power chord damage. This is Sona's w, it heals you and an ally champion with the lowest percentage of health. It also grants bonus magic resist and armor for a short time to both of you. This aura for this ability provides bonus magic resist and armor to allies. The bonus is the same for both the active and the aura but the actives adds up with the aura. the power chord portion of this ability decreases a percentage of the targets damage out put for a few seconds. It is useful to target enemies like the ADC. This is Sona's e, it provides a speed boost to your and nearby allies. This also includes minions. The aura gives a lasting speed boost to nearby allies and adds up with the active effect. The power chord portion of the ability changes Sona's next basic attack so it slows the target hit. This is Sona's ultimate and it deals magic damage while it stuns enemy champions in a line. Sadly, you have to aim this ability :(. It is very useful in teamfights where it can stun the entire enemy team.

Basic information about item buys are in the notes. Here, I will talk about when to buy which items and other information. Athene's is for when you want to be more aggressive. Mikael's is for more passive supports and its active is basically a second heal with a cleanse effect. Ionian Boots of Lucidity: Cooldown boots is useful for casting spells more frequently and with the mana regen items stated above, you can spam your abilities even more.
Mercury's Treads: These boots have a nice MR boost and is good if you find yourself being CCed alot.
Ninja Tabi: These boots are good if you always get auto attacked by the enemy. With Frostfang, I do not upgrade it to Frost Queen's Claim often. As said in the notes, the stats provided are not very different compared to Frostfang. The stats only have a slight boost and I find that the active is very weak with 50 damage. The slow is still useful but better can be achieved with your Passive+E and Exhaust. Again, the choice is up to you.
Offensive Build Vs. Defensive Build
I sometimes go with an offensive build because I always poke at the enemy every chance I get, but I go with a Defensive Build if the enemy is just dominating my team or if I die easily. (But another reason for dying easily is making mistakes and charging into battle) Most of the time, I go with a mix between both.
The build starts out the same with getting Frostfang and Ruby Sightstone. From there, I build Athene's, Lich Bane, and Zhonya's or Locket of the Iron Solari. I buy cooldown boots when I feel like I need them.

Here will be information about the laning phase and safe spots to be.
With every other champion, make sure you are in a safe position. For aggressive play, try to be in a position where you are safe but are still able to poke and harass at the enemy. A good spot is the bush on the bottom side of the lane. Which bush you go in depends on where your minions are. If they are leaning towards the enemy, go to the bush closer to the enemy. If the minions are near your tower, then stay in the bush near your tower.
For some more passive play, try to stay away from the enemy, but make sure you can still get experience. Also try to heal as much as possible with your Aria of Perseverance but pay attention to your mana bar. Regardless of your play style, try to give the most to your ADC. This can be achieved by blocking skill shots for your ADC. Dodging them would be preferable but in cases where being hit is inevitable, better you than your ADC. A good example is with Caitlyn's ult. If she targets your low health ADC with her Ace in the Hole, you should jump in front of them even if it kills you. It is important for your ADC to be alive and able to farm or kill.

IMPORTANT NOTE:

It is not only the support's job to ward. EVERYONE should be warding. With a few exceptions at times such as with the ADC who should be focusing on their main build. But with the addition of trinkets, everyone can ward at no cost!
Unrelated note: YOU GET A TRINKET! YOU GET A TRINKET! YOU ALL GET TRINKETS!
ok... I'm done with that
Back to warding, try to ward at big choke points where you have to pass in order to get to lane. This includes the tri-bush, the exit from blue buff, and in the bush right outside of lane. Since you are only allowed to place 3 stealth wards, your ADC can use their Stealth Ward to help out and ward the river bush.
Later into the game, you should ward baron and dragon, which become important objectives to keep track of. Trailing off into late game, baron will become more important than dragon and most wards should be focused around on baron. I would recommend placing a Vision Ward near baron just to eliminate your opponent's sight of baron. Sometimes, it can be fun to have ward wars with the enemy team. Even though it does not provide anything important besides wasting gold, its just another relaxing part of League of Legends.

Thanks so much for taking your time and reading my guide!
If you have any comments or questions, please write them down in the comments section below!
Any suggestions and feedback would be helpful too!

Update 2/24/2014: Thanks to EdisonKhoo for your feedback!
Update 2/27/2014: Added some coding to make the guide not to plain looking.
Update 3/2/2014: Added more information to runes.
Update 4/13/2014: Added laning phase, warding, and abilities.