This is my second build for Mobafire (my first being one for Karma) Please remember to try this build a few times and read it thoroughly before voting. If you down vote, please tell me why.

Fairly straightforward runes. I personally have a hard time last hitting with her, so I take gold/10 quints. I'd suggest this even if you are good at last hitting because it means that you'll be able to allow you lane partner to take more last hits while still getting gold. More gold means better items.

Marks make you spells hit harder.

Seals: I go for straight mana/5 seals instead of mana/5/level because sona has a REALLY hard early game as far as mana regen, and while this will continue throughout the game, it is decently mitigated by items and summoner spells.

Glyphs: These in combination with masteries will give you a full 15% CDR at level 18. In combination with items you will quickly approach the magic 40% which means more aura spinning.

The offensive masteries up your damage with you abilities, and the Utility masteries help keep your mana up as well as start generating you gold and cooling down you summoner abilities. Fairly straightforward I think.

NOTE: DO NOT PUT 3 POINTS INTO INTELLIGENCE! It will be wasted by about level 10 as your runes cover it.

If you don't want to put 2 points into Perseverance then put a point to up monster buffs. Also feel free to switch your summoner spell buffs around to your liking. (to boost Ghost or Teleport or Flash as you like)

Ok this is where I expect to get a lot of flak. Mostly from the Starks Fervor. Let me give you my reasoning:
A: Sona's job is to provide auras that help the team.
B: Sona's job is to keep teammates alive.
C: Sona's job is to debuff enemies when possible.

Let's see how this item accomplishes these tasks

A: Starks Fervor provides bonus attack speed and lifesteal to all nearby teammates. +20% attack speed on your carries will go a long way, especially when you complete this around level 13-15.

B: 20% lifesteal. many people will say "But if you just get AP her heal will be stronger!" while this is true, 20% lifesteal on you AD carry will heal a LOT more than most AP items. a good AD carry should be doing at least 150 damage with an attack speed of at least 1.5 attacks per second. That's 225 DPS. Now for sport, let's give the enemy enough armor to reduce the damage by 40% (their prime targets won't have that much) that's 135 DPS post armor. 20% of that is 27 healing PER SECOND. Sona can cast her heal at most once every 4 seconds. To be on par with this rate of healing the AP item must provide enough AP to heal and additional 108 HP. Her heal increases at .5 AP which means you must get an additional 216 AP from a SINGLE item to match this. Also the lifesteal applies to EVERYONE not just the two lowest health champs like your healing ability.

C: -20 armor aura? go AD carry! deal that damage. Also makes tanks less tanky.

now that I've got that reasoning out of the way let's talk about item order.

I start with aMana Manipulator to help my lane mate with their mana issues so they can spam their abilities and help provide myself some much needed mana regen.

I follow this up with aPhilosopher's stone to provide some more mana regen and start generating gold.

Next is CDR boots to increase speed and aura spamming.

The next item I get is dependant upon who your teammates are, and how the game is going.
A: Game going poorly, enemies keep escaping with little HP:Aegis of the Legion

B: AD heavy teammates/lane mate:Starks Fervor

C: AP heavy teammates/ lane mate:Will of the Ancients

If you didn't getAegis of the Legion yet, get it now, otherwise get based on your team.

If your team is HEAVY AP and little to no AD, then you may want to consider gettingAbyssal Scepter unless an ally gets it already.

Finaly finish by upgrading yourPhilosopher's stone to aEleisia's Miracle.

Start with Q, a point in W at level 2 and put a point in E at level 4, leveling your ult when possible. From there it depends on how your lane phase goes. If you're doing well, up your Q and keep the push. If you're playing defensive, up your W. I tend to alternate between them, leading with Q. Aside from that, keep track of your passive building up and use it with Q active for harass, or W active for Ganking/fighting.

Also note: Your ult has a minor cast time, so be careful when using it on fleeing enemies as they won't be hit if they're on the fringe.

Clarity is almost required with this build. You WILL run out of mana, and I've often found teammates run low after team fights. Imp Clarity if used wisely will remedy that. Now this doesn't mean you should always save it. With your improvements it has a relatively low CD (About 3 min.). Don't be afraid to use it during the laning phase to stay in lane longer. Also make sure your partner knows that it's improved and you're about to use it. Then they can spend some mana on harass that they might otherwise avoid.

Clairvoyance: I like this as a bush checker, and to keep tabs on the enemy jungler. It can also be used to check Mia's when called to help your mid avoid ganks. With improvements reducing the CD to about 42 sec, and revealing for 10, you ought to be spamming this to keep your team map aware. Note: It doesn't reveal stealth.

This build focuses on providing Aura's for your team. You will get focused when the enemy figures out what you're doing. Ask your tank to kindly guard you and be sure to hang back. Your damage has a decent range, and your team will need your heals. Play defensive and remember, you're a support. You won't win the game on your own, but you will make it outrageously easy for your teammates if they are competent.