Not Updated For Current Season
here's how I played Sona the first games after I got her.
First of all, Sona is a great support champion, but that does not mean you won't get kills with her. To get kills, you'll have to get a lot of AP, and that's what I will do in this build.
I start off by buying Meki Pendant for mana regeneration, because you'll be using your skills a lot. Go buy Fiendish Codex or Tear of the Godess asap, the order you can choose yourself. (Tear of the Godess gives you the mana stacking thing, Codex gives you some AP fast) After you got those two, get some boots. I always took Sorcerers 'till now, so your first skill will hit harder.
After the boots, upgrade your Codex and Tear of the Godess to Deathfire Grasp & Archangel's Staff. (both AP & mana & mana regen related) Then you continue with Zonya's Ring. I haven't been able to play long enough to get past Zonya's, but I guess something like Rylai's Scepter would be a good idea, so you get some additional health. Anything with AP will do I guess.
I use Clearvoyance, it's very important that the enemy doesn't get into you, because there will be no way out if they do. Your task is to have a good map awareness, so you can guide your team to victory. Check bushes and buff monsters frequently, it will also get you a lot of additional easy kills.
Second, I use Teleport, so you can get back to your lane fast in early game (after you went restoring mana or buying stuff), and get to the right spot (near other allied champions) later on, so you can start healing/buffing them.
Now, how do you play Sona?
I prefer playing a side lane, not mid, but I think Sona can easily handle mid if you want.
In early game, harass your enemies! Get hitting creeps far out of range, and when Hymn of Valor is ready, move closer, hit A and back off. When you or your lane partner gets hit, heal immediatly. I tend to max out Hymn of Valor first, so you can harass better... If you want to play more on buffing & supporting, you may better max out Aria of Perseverance first. Try last hitting creeps as much as possible with Hymn of Valor too, so you'll earn some cash. You got to make some trade-off between hitting enemies and killing creeps, so choose wisely :).
Something that you should think of, is the magic bonus damage after every three skill you use. It gives a pretty good bonus damage on your next standard attack, so try to harass your enemies with it again!
When you get your ulti, time it well, especially if your still in your lane. It gives a short stun, but it can be enough to kill an enemy, if your lane partner moves in on the right time. When you time your Hymn of Valor good enough, you will be able to get the kill (although an assist is good too ;) )
Mid game & Late game
Not a lot of difference for this sexy champ, just move with the group and buff them! Heal as much as possible and kill creeps to get some money. Don't forget to use your speed buff, it gives your team a great advantage while pushing a lane. Your ulti is very powerful, it can stun multiple champs in a team fight, it can stop some ulti's (like Katarina for example), or it can stop runners.
This is how I played Sona 'till now, I hope you've gotten something out of it!