First of all, this is not an "optimal" guide. The current meta suggests that Sona should be played very passively and should feed (IE give up exp/gold) to her laning partner, buy all the gold/5 items (to make up for her lack of farm) and then as many aura items as possible. I've tried that, and it works. If you want to be competitive in high ELO ranked, go for it. This guide is for normal mode and ranked mode up to about 1500 (highest I ever went). Following this guide allows you to have a very good early game and then go from there. Sona is a very flexible champion, and depending on your early game, you can decide what to buy from then on (the item-build is just a template, nothing is set to stone)

The runes are also flexible except RED imo. Yellow could be the mpr/lvl or flat mp regen. I like the early oomph from flat mpr because i plan to get a tear early so mana is only a problem early game (not really actually). Blue imo should be CDR, but again up to you. Finally quint...i suggest CDR but flat HP works and so does movement speed.

Masteries...9/0/21....the utility is pretty cookie cutter, change it only for summoner's spells.

Summoner spells. Flash is a lock IMO. Sona is a very burst champion, sometimes a flash+Q+power chord would be enough to finish someone off. Flash is also very very good for positioning a perfect stun. OFC flash can also be used as a getaway. Since i encourage boots of swiftness, once you put a little distance between yourself and the threat, the boot and speed-aura would easily outpace everyone. Ghost is to make you more survivable ofc. CV is the strongest choice. You should always have it because you're the support. Unfortunately i'm at around 1400-1500ranked and I play some normals where there's either no junglers, or they don't understand timing. Again, if you're gonna be competetive at a high level, go CV. If you don't want to waste a spell that is useful in mechanics, but no one is able to capitalize with it, go with ghost.

Sona's Q always prioritize champion before minion, so you only have to make sure they are within range. Her moves does not require you to aim or face in any direction meaning you can dmg ppl chasing you. Turret aggros on you when you deal damage not when you cast, but her Q has a delayed travel time, therefore, you can easily run in, Q, run back while the dmg wave is travelling, and then the Q lands. This makes poking very safe as you're can release the spell and get out before the turret shoots you in the face.

Sona's W has a pathetic CDR. It is made up by the fact that it heals yourself as well essentially making it 2x more effective. As Sona, you should be at bot with a partner.
Ideally, you'd want a strong one like Caitlyn, who doesn't take a lot of dmg herself and is a great pushing. Alistar is also a good one. Anyone who perfers sustain over burst compliments Sona, but Sona compliments everyone.

(Note: Just because Sona has a heal does not mean your partner should dive in. Her heal is most efficient if BOTH of you are dmged making the most out of it. Her heal is frequent but very weak. Ideally your partner is ranged or tanky like Alistar who can poke a bit, take a bit of dmg and get healed by you. Your mana is best spent using Q, now W, until team fights. W is a hp/mp balancer. Use it when you have 60% hp and 100% mp.)

W is also auto target which is good and bad. Good as in its easy to control and automatically target the lowest % hp ally. Unfortunately, sometimes a tank with half hp does not need your little 2-300 heal as a carry who is at 51% hp. Keep that in mind.

E is very underated. Having superior speed means that w/o a snare, you will survive if you get pass the auto atk range. It is not recommended to spam until you have a reasonable amount of mp from tear.

Always abuse the fountain. When you spawn, learn Q, then use it. Buy your stuff and make your way to the lane. Your Q would be up before you leave the fountain. This essentially gives you 2 stacks of Q before you engage. This is important because that means you can last auto hit minions without wasting your charge (if your lane enemy is still hiding in the bush), but you can have amazing burst by using Q->AA->Power chord (if they pop up). Once you get tear, every B= use as much spells as u can in the fountain while you buy.

Generally Q is the best powerchord modifier throughout your game. When in doubt, use Q. W is useful in a team fight because the -20% dmg reduction is major on their primary carry. E is useful for chasing down enemies. Considering that you're in the back of the fight most of the times and having boots of swiftness, you have a good chance of running from a losing team fight. Turn back (if you're confident) and slow power chord the enemy that's chasing your carry, this is very very important.

Ulti. Of course, the optimum use is if all 5 enemies stand in a straight line and u catch them all. This is rarely gonna work if they're not totally stupid or in a jungle. 2-3 is a good catch. Early game, it is worth using it on one person. Don't be afraid to use it, the cooldown isn't very long and you should have decent CDR anyways. The ulti is useless if you die w/o using it. It's also useless if you hold on to it too long because either your team is already owned therefore there is no one to pick them off even if you penta-stun. If your team is already winning, you might as well save it then. So ulti is reallya judgment call.

In normal games, there is rarely any junglers and nobody last hits, so this is where Sona shines. In the beginning, nobody really has any AP/AD. Your buff to both you and your ally, a strong Q, and a free power chord would always give you the upperhand. Do not try to burst them down before minions spawn, it's better if you poke (only characters like trist and cait who have absurd AA range can really out harass you).

In ranked games, you should only Q to harass. Last hit minions (if you're below 1500, don't worry about "stealing minions". Many of them can't CS properly anyways, might as well go to you then have it wasted). Remeber, W, is very very uneffective, try not to use it unless u have to. You should consider leveling W at level 3 if you're getting massively outharassed or your teammate thinks hes invulnerably just because you have a heal button. The reason why i don't suggest getting your E until lvl 5 is because: at low level, you mainly use E to rack up tear stacks, your Q is too weak to make good use of your chasing ability, level 2 W is 2x base of lvl 1 W, and by level 4 you will take some damage.

There are a few magic numbers you want to reach before you B: 605, 955, 1105. Tear, Tear+boots, Tear+boots+2ward. Usually you will have enough for tear+boots around 6-7ish so that is going to be the time you B most often unless you score a kill or die. If you do manage to score a kill, it might not be a bad idea to start racking up your tear early with an early B. Also consider B if you overextend or run out of mana. Your goal is to get a tear,lvl 1 boots, and hopefully a blasting wand and even finish your AA staff if you have a good early game before turrets starts to fall.

I recommend sheen for your 3rd item because: it's component items are great, it makes up for your horrible consistent dps, amplify your burst dps, works well with your low cd spells, and help you build a LB later.

At this point, probably a turret or two has fallen. Remeber that Sona is strongest when she is with allies so if you lose your turret or wins one, you should help the other lanes (perferably top). Don't spam your auras if no one's around because you can't afford to do that yet, but when you engage in 2-3v2-3 skirmishes, you should be able to roll all 3 auras for a long time. Remeber, always poke poke poke. You want to win by attrition. Mana is on your side meaning you can sustain, you have a good poke, you have an alright heal (now that you have some AP from aa staff) to help your allies sustain, and if they commit, you can then ulti and burst. This is the time where you should start considering your 4th aura/defensive item

Here there are usually 3 choices I usually run with. Shurelya, Soul Shroud, and Morello.

Shurelya if neither side has a distinctive advantage. Don't estimate it, it is very powerful especially in late game. With boots of swiftness, you can always get away unless you get snared, and it gives you the option to swoop in, buff your melee allies and then you can both flee. Offensively, it allows your team to get into position (such as creaming their carry). Shurelya itself has decent hp/mp stats.

Soul shroud is a strong choice, best if you have champs that can use it's CDR well like lux. I buy this when im not doing great because of the flat hp. At this point, if you're not doing great, you should start building more team friendly items and try not to die over being offensive because if you don' by now, you won't be able to snowball into lategame anyways.

If however, you get maybe 3/1/1 or 2/0/4 or something nice, you should consider getting a morello. The AP is nice, but the CDR is where the money is. This allows you to rotate your spells much quicker and build your AA stacks faster. Basically this allows you to continue your advantage and keep snowballing.

Upgrading your boots first or completing your 4th item is entirely up to you. The reason why i didn't went with the -CC boots its because you're not supposed to be in the front lines and you have great surviving moves ala heal/ulti/speed/snare. The CDR reduction is also unnecessary because you will eventually get some from other items and you don't want to put yourself in a position where blue buff would put you over the cdr cap thereby wasting stats. Boots3 along with speed boost and snare makes you a kiter. Considering that you should always hang in in the back unless you want to Q+Power chord, this increases your survivability and when things go right, you will be the first back into the fray to snare. These boots also allow better positioning for ulti (in fact, the only important thing about Sona is positioning. How to ulti, how to isolate yourself from the ones you don't want to heal and to the one that you do. How to stay in the maximum Q distance).

Don't upgrade you sheen to LB until you have built your 4th item. You don't have enough AP to make it worthwhile yet.

At this point it should be obvious whether or not you're winning. If you're doing well, get a LB because at this point your AA should be almost maxed (from constant spamming). Your AA would allow you to roll all 3 moves w/o mana problems.

If you are losing, i recommend getting a soul shroud/shurely (whichever one you did not buy).

An hourglass is always a solid choice as it gives you some armor and a very useful active. Sometimes you may find yourself being focused (which is logical because you might have a good early game, and your opponent thinks you're gonna snowball hard, which you did not because you spent some money on an aura item and with 1 dmging spell, you just don't scale that well). Remember, like 30-40% of Sona's effectiveness is just being alive. Your 3 auras are still up when you are invul. Hourglass allows you to wait for a specific spell to finish it's CD. This is important because you may have 2 stacks of power chord and you see a low hp ashe but your Q is on CD. Invulnerability also allows you to regather your thoughts and make the right decision (such as ulti)

At this point rabadon is just icing on the cake, the game is usually finished by the time you finished your zhonya anyways. It might actually be better if you use your money on Blue elixir and red elixir instead.