Read first! This guide isn't completely finished, but since she's in the free champion rotation I publish it now for you to read! It can be used for normal mode and dominion, at the moment it's written for dominion in special. But everything works the same in normal mode!

Summoners, I welcome you to my very first guide for League of Legends.
If you're wondering why I am writing a guide, especially for a supporter champion in dominion, and even if you're not, I feel free to tell you why: A friend told me that supporters were completely useless in dominion. But I don't feel like agreeing with him.
It may be true that, left alone, a supporter is rather an easy target and can't affort much. But therefore you're a teamplayer, and mid-game you'll be able to defend a capture point on your own - for a limited period of time, because against more than two enemies you just can't stand a chance.

In this guide, I'll show you how to build and play Sona the way she's able to deal proper damage, keep her team alive due to her support and really be useful in dominion.

First of all, there are several points to meantion about Sona. You should check them before deciding to play her, you should be able to cope with them at least.

Pros:
- rather easy to play
- good supporter -> constant heals and buffs
- able to deal some real damage
- beautiful sounds ;)
- pretty good escaper due to heal and speed buff

Cons:
- squishy troughout the whole game (but you are able to escape)
- main target in most team fights
- sometimes the little heal just isn't enough for the moment
- not able to defend a capture point on her own for long

Sona is a caster, so I went the 9/0/21-way.
Be sure to take the Archaic Knowledge magic penetration from the offensive tree because it's very helpful to max out your damage potential.
In the first row of the utility row I took Good Hands for being back earlier after dying, one point in Ghost because moving around very fast can be very useful in some situations and only one point in Preseverence: in my opinion, with your heal you don't need too much health regen, and your mana regens fast enough due to the Tear of the Goddess. But still it's more effective than Expanded Mind because your mana pool is already big enough.
The next row should be clear: you already have enough mana, so go for the experience from Awareness .
The third row depends on your own preferences. I love getting extra money, so I went for Greed and the mana regen from Meditation is in my opinion more useful than keeping buffs up longer or sharing the bonus from Clarity with teammates (because you won't need this summoner spell). You can vary here, and putting the one point from Preseverence in Utility Mastery is one good possibility, too.
Next should be clear again, extra speed is always good - especially in dominion - so go for Quickness , and because Flash can save your life take Blink of an Eye .
Then of course you take Intelligence because casting is everything to Sona. You won't use Clairvoyance so don't even think about Mystical Vision .
Last is Presence of the Master , reducing the cooldowns of your summoner speels - take it and enjoy more Flashs and Ghosts.

There are different ways to supply Sona with runes. But in my opinion, there are only two different possibilities that are making real sense.
But let's start with the sure things: Greater Mark of Magic Penetration will make your attacks more useful, it should be logical why I choose these ones.
For the Quintessences, take Greater Quintessence of Ability Power because the bonus ap will help you in early game and so on, I think you know about the advantages already.

Now there are two decisions for you to make:
1. Ability Power Greater Seal of Ability Power or Life Greater Seal of Vitality?

It mostly depends on your own preferations. If you tend to die a lot, think about the Vitality Greater Seal of Vitality and if not (or if you don't care) take the ability power Greater Seal of Ability Power. I took the ones with flat ap because the stacking ones are starting to become better at level 13 - many games end at or even before that point.

2. Ability Power Greater Glyph of Scaling Ability Power or Cooldown-Reduction Greater Glyph of Cooldown Reduction?
This is the more difficult decision. Sona should be casting all the time, always switching the auras, so having little cds allow you to use the spells you need when you need them and heal more often. Here you should take the flat ones as well, same reason as above. On the other hand, you can boost your spells with ability power: but then go for the stacking ones, because they will be better than the flat ones at level 6 and that's nothing in dominion.
It's your choice, depending on your playstyle. At the moment, I use the Greater Glyph of Cooldown Reduction because I bought them for Katarina before. But going for Greater Glyph of Scaling Ability Power could be a good way, too, I'll try that and tell you about the results. You can do that as well, just leave a comment and help providing opinions for the best choice!

The choice of your summoner spells shouldn't be very hard. I always take Flash and Ghost in order to move around quickly and save me life if my speels aren't enough. In my opinion, movement is almost everything in dominion, and these two help you to be the first at your goal.
Of course there are a lot of good choices instead of just movement. But consider that you are mainly a supporter, so leave things like Exhaust and Ignite to the dps. Garrison could be a reasonable choice, but I found it very difficult to time and find the very right moment to use that spell. But if timed well, it will secure some kills under the enemies capture point or help you defending your own.
No need for Teleport in my opinion, and you won't need Clarity: your mana regen is enough due to the Tear of the Goddess. You will have to recall or die before running out of mana.
Well, what's left... Clairvoyence is useless for my uses, Smite as well. Promote isn't as useful as Flash and Ghost... Cleanse won't help you much because if you're getting cc'd your almost dead no matter what, you have your own Heal with you all the time (this one will even weaken your Aria!) and Revive isn't needed because the respawn times are rather small in dominion.
...so decide wisely. You know my arguments and opinion now.

You start your game with an amount of 1375 gold. That allows you to buy your Tear of the Goddess and Boots right at the beginning.
You should trigger the passive from your tear before the game starts: for every spell castet you recieve 4 mana. Doesn't sound much, but you have about a minute time before your spells will cost mana, so use this time and enlarge your mana pool.
Next are your shoes, Ionian Boots of Lucidity. They will decrease your cooldowns, allowing you to spam your spells. Sorcerer's Shoes would be an option, too, but only if you want to follow the damage-path. I think you can choose your shoes according to which runes to use: If going for Greater Glyph for Force, take Ionian Boots of Lucidity and if you choose Greater Glyph of Cooldown Reduction maybe you don't need even more cooldown reduction and can buy Sorcerer's Shoes. I'll test that once I have the Greater Glyph for Force and add the information here. 'Till then it's completely depending on what you favor.

Your next goal is Sheen. If you can only buy one part, I would go for the Amplifying Tome instead of Saphire Crystal because I never run out of mana, and if I do, the higher mana regen in dominion fills it up again. Back to Sheen: It will double the damage of your next attack after every ability you use. And since Sons is spamming spells all the time, nearly every of your autohits will deal doubled damage.

As soon as you can, build your Sheen into Lich Bane. It offers you your real damage potential. After every spell, your next basic attack will deal an amount of 100% of your ability power as bonus damage. Well, this effect has an two-seconds cooldown, but that's nothing you should worry about. It activates often enough, and combinated with your Power Chord, it unleashes unknown power! Just for imagination: On Level 18, one physical attack using Power Chord and Lich Banes passiv would deal: 101 basic attack damage + 176 bonus from Power Chord + 482 bonus damage from Lich Banes passive = 759 --> and this amount is doubled when Hymne of Valor was your last spell! That makes an autohit damage of about 1518. Crazy, isn't it? (The data is from Mobafire as you can read in the table at the top of the guide, if I'm making any mistake please tell me.)

Now you can build Archangel's Staff from your tear, giving you bonus ability power in high of 3% of your maximal mana. Doesn't sound much, but it actually is. On level 18, it would be around 85 AP. Not too bad, isn't it?

The next item depends on your situation in the game. If you're getting much to much damage and you're dying a lot, go for Zhonya's Hourglass. It will give you armor, ability power and an on-use effect that makes you immune to damage for 2 seconds. You can't move, but if your team turns the fight around it can save your life and allow you to help them furthermore. Otherwise, if you're rarely attacked or if you don't mind being hit or even killed, buy Rabadon's Deathcap for even more ability power and damage + heal output. I don't think I have to say more about this awesome item.

If you decided for one of them and bought it, you can next go for the other one. But if you took Rabadon's Deathcap and still think you don't need the armor, continue directly with Void Staff for even more ability power and 40% magic penetration. The staff is a great item to maximise your damage.

That is basically my item build, maybe I will add some other possibilites later.

In dominion, you start at level 3. In order to perform a good start, you should skill all of your three spells. The Song (speed) to be first at the capture point on top, the Hymn (damage) to help scare pf the enemy and the Aria (heal) because you WILL need it.
Sure it's possible to skip on of the skills to make another more effective, that depends on your playstyle. I could imagine leaving out the song in order to amplify one of the other skills, but then the whole team would loose time that is really needed at the beginning...On the other hand, leaving out the damage in order to heal more or be even faster would be a possibility, but due to the cooldowns I prefer having them all - after all it's your personal choice.

After the game has startet you have to choose the way you want to go: Damage or Heal?
In this build, I used the supportive way and skilled the Aria whenever possible, followed be the Hymne. In my opinion, Sona is still meant to be a supporter so it's more usefull to heal more than to do a little more damage. But if you want to go for a caster-dps, try it and max out the Hymne first. Leave a comment an tell me whether the damage was worth the loss of heal. ;)

I always max out the Song at last: of course moving around fast is very important in dominion, but even one point in that skill is better than no movement-speed buff and the other auras are more important to me.

And don't forget to skill your ulti whenever possible, but that should be clear I think.

Let's get started with the real part: How to play Sona properly ingame!

Basically, there is not much to say about. Sonas cooldowns are pretty low, so it's easy to switch from one spell to another. You can do that every 2 seconds I think, and that's what you're supposed to do. Never stop casting! You always want two auras to be active and everytime you cast you'll do another useful thing: speeding up during a hunt or escape, healing your partner and give him more resistance or dealing damage and increase the attack power of everyone around you. So keep hitting your Q, W and E buttons depending on the situation.
If your fighting an enemy near a capture point, change between heal and attack. While moving between speed and heal (if on low health) or attack (if you want to harrass a chasing enemy). After a while, you'll get a feeling when to use which spells.

So much about your part as supporter. If you decide to deal damage, there are two things you have to be aware of:
1. trigger your passive, have Power Chord ready after a cast of Hymne of Valor.
2. pay attention if the passive of Sheen/ Lich Bane is activated
If both conditions are fullfilled, auto-hit a nearby champion and see how much damage that will deal!

Now let's think about Sonas Ultimate, Crescendo.
I think it's pretty hard to time Sonas ulti correctly, so I tend to use it way to rare. My advice is: it's better to use it and don't score the best effect then to forget it!
Your ultimate should be used whenever you can hit more than one enemy and give your team the chance to tear them apart. If your on the way to a turret being captured by the enemy with your team, use your ulti to cancel the capturing from the most enemies possible and to make them easy prey for your team. If you're defending a capture point on your own, don't be afraid to use it against two or even one enemy, best in range of the turret to keep you save and give you (with help from the turrent) the chance to kill them.
There is one important point about your ultimate I want to warn you about: Don't use it against Trundle...it's stun is useless against him and without, you'll probably be not able to kill him on your own.

These are the best advices I can give you. The important point is that you have to get these things into mind, know when you should do what. After a while, you will know what to do by yourself, this is just to give you some guidance and call things into your mind which you weren't thinking about before.