Allright, this is my second build for Sona.
Please read my guide through before using it as you'll need to know what you're supposed to do with this build.

Sona can be played many ways. While full ap seems most obvious on Sona she still has to rely on her team. She still can't do it on her own because she is meant to be supportive. Offensive Sona is okay but keep in mind that any problems might show up if your team isn't that reliable.

That said, it's better to get your teammates all buffed up. As these will be far more viable to get very strong if they are the offensive players. My skill order stays the same from the previous build. As for the items I'll take some more convient for champions that don't get many kills, and a few minions.

I will first talk a bit about my reasons for runes, pro's and con's and my item choises.

+ Good at harassing early game
+ Valuable champion at every moment of the game
+ A stun that can turn the tables
+ Heals that also gives resistance and armor

- Still squishy and mainly targeted due heals
- Not many escaping abilities
- Not very strong at dealing damage late game

I chose these runes specifically for the early game, as well as this gives just the right amount of AP to keep your team going. In early games you go harassing and will need the AP to keep your enemy from gaining minion kills by letting them shiver from the damage you do. As well as the mana regeneration is quite important early game to keep on harassing.

Seem pretty obvious to me, many people forget using Clairvoyance alot while this is one of the best Summoner spells on Sona she can have. You can scout area's and can see any enemies lurking in the bush trying to let you draw close. As this spell can be casted anywhere you can check for everyone for any ganks that could be coming. Many people forget to use it and that is is quite a waste for a spell with such a short cooldown. People can not hide in the bush while you keep your eye on them with CV as well as this makes it easier for you to harass them since your Hymn of Valor can only be fired at them if you can see them. This helps Sona alot forseeing incoming ganks coming towards her, as well as being able to point out where junglers are wandering.

Flash is obvious, it's for escaping ganks mainly. A real must on Sona with her lack of escaping skills. Song of celerity cannot save you as most people have flash with their summoner spells and Sona is not the fastest champion you can imagine.

Very obviously is the first skill. Hymn of Valor raises your damage as well as your teammate's. This skill is your all important early game harassing skill. However as described by many people, Hymn of valor aiming champions more likely than minions? In all games I've played with Sona I never ever saw something like that happening. Hymn of Valor reaches for the enemy closest by Sona, not the champion standing behind the minion.

Aria of Preservance is a must for keeping lane control. But if you're harassing pretty well you'll see how much the enemy will hold back. Afraid to get hit by the Hymn of Valor. Watch as you try to draw close they immediately back off. However Aria is pretty much important anytime. If the enemy does actually dive close ór manages to get a hit on your partner you'll need to fix him up too. If it's only you to get hurt ealy game use your potions instead of using up the Mana you'll need so much for harassing your enemies.

After raising hymn of valor twice you can put one skill point in Song of Celerity. Don't use it too much early game, as you don't really need to run away. And it's too early to go turret diving unless the enemy is at very low health of course...

Cresendo is very important, get it instantly. It is a life saver, holds enemies that try to flee and very useful when they wage a turret dive at you. As the turret will focus them and they are rendered helpless.

Utility is the most important tree for Sona. Not only are her Summoner Spells buffed, but she needs all the mana regeneration she can get, as well as a lot of experience to keep her level up, as she can't run off on her own farming for some experience. First priority is to help your team by keeping them alive and let them get kills.

Clairvoyance will stay longer active and the cooldown is reduced, so its very important, as well as a cooldown reduction on flash. As for the offense I prefer now for the early harassing point.

Starting off with wards is neccessary. Wards are one of the most important items Sona needs to keep the control in her lane without having to worry about junglers or people going SS. So constant warding should be done. You can either choose to spend all your money out right at the beginning of the game by buying some more wards than I prefer 5 at maximum. As I stated at the summoner spells chapter I said you can use CV to beat the enemy out of the bush. However, if you set up wards and save the CV for your allies in diffrent lanes then you won't be only supportive for your own lane, but also the other lanes and for your jungler if you have one. This also comes in handy when junglers are skipping around and you can CV them to see where they are walking. Late game, you go for the sight wards and put these at Dragon and Baron. Why the sightward? So that you can see if the enemy team warded them too. Keeping the Baron and Dragon under your control gives you a small advantage.

Potions are mainly for the early game, late game you wont need them since you'll be busy healing your team. And if you really get hurt you usually die before you can even touch a potion. Early game these come in handy, you can keep on laning and won't need to waste mana for selfheal.

The faery charm is your first manaregen item, since it's pretty cheap it's the item you should buy. Once you got more money buy the philosophers' stone on your first travel back to base. Later on you'll upgrade it into Shureyla's Reverie. Combining this item with the Song of Celerity will give the massive speed boost you'll need to chase or flee.

Heart of gold is the reason to give the minions and kills to the ally walking along with you. You won't need money since heart of gold generates it for you I suggest upgrading this item as late as you can into Randuin's Omen. Although if you get the chance to get such an expensive item in the end, take it. As it is one heck of a good support item that has the same effect as exhaust but then on scale of targeting their entire team.
Optionally is combining the heart of gold with Kage's Lucky Pick for more gold generation and some AP. The reason I choose Heart of gold first rather than taking the lucky pick earlier than the Heart of gold is because Sona needs the life to have more survivability.

The ionian boots... A great item when it comes to cooldown. Always go for this one. But if you find yourself getting targeted a lot mid-game that is the only reason to go for Mecury's Threads instead. You'll need the cooldown to support your team better as well as better cooldown on your Cresendo.

Now that we've got our Core items complete we'll go for some more luxury items. Mind that these are opotional items. The last three items can be in any order you want or be changed into items I suggest later in this chapter.

Ageis of the legion, works well together with your healing spell giving a big defense rate for your entire team. Good item for raising the defensive capabilities of your team.

Banshee Veil, used for taking that one shot that might CC you. Also stops a few champions that cannot kill you without using all their abilities. (Lux, Morgana...)

Soul Shroud, very important aura item on your team, as it boosts cooldown reduction and mana regeneration for your teammates. An Ideal item this one is.

Another opotional items is Quicksilver Sash that can compensate for your Mercury's Thread

I do say Rylai's Crystal Scepter is an useful item as it gives AP, Slows enemies with your Hymn of valor and also gives a nice amount of HP I prefer the Omen as this one does not hit two champions at maximum, but five. As well as they get weakened by it. And it has nearly the same range Hymn of Valor has and also having some regeneration.

Sona can combine with various champions.

If you team up with katarina it could end up mid game getting katarina feed very well as the enemy usually underestimates Sona, trying to get a kill off her only to find out it was a bait to let them draw close to her. And since you hold back a bit katarina can use you to make a quick get-away from the enemies.

Teaming up with stealthed players is possible. However, don't use aria of preservance while your ally is stealthed as this kinda gives away where your teammate is walking since the healing beam just follows your friend and can be countered with skillshots. Same for partners hiding in the bush. (Also note that if you both are in the lane, both go in the bush, you really have stupid enemies if they don't notice you're nearby while your ally is obviously just killing minions with Sona's aura up.)

Carries are best to get along with. But keep in mind some people playing carry get very confident when they walk along with a support. They will push harder than usual, forget to keep an eye on the map for possible ganks. Because they think they are pretty much invincible with a healing champion around. So if you ever get an Carry with you that acts like this pay close attention for ganks, ping the ganker whenether he's there, and tell them to go b whenether they are on low health.

If you are laning with a roamer make sure to be careful. This is the point where you're allowed to farm for some gold since your ally is most of the time away. Try not to push too far and be careful. They know sona isn't one of the strongest champions and makes them pretty eager to go turret diving. Use these oppurturnities to stun them and kill them with help from your turret.

Early Game
Right at the beginning of the game you'll charge up your Power Chord right away by using Hymn of Valor three times in base. You can choose to throw your CV in their base to look at their team setup but keep in mind that the enemy can see you using CV and pretend to go somewhere. Put up your wards around the fifth minute, do it earlier if enemies are SS. Junglers will first go for creeps in the jungle grabbing some items first before going into battle. Check out the enemy lizard elder. If it's gone the jungler is about to gank someone. With hymn of valor you'll keep your enemies in line. Early game, you use your Hymn of Valor ONLY for hitting the enemy, not for farming minions. You'll need the mana for the harass since Sona is quite mana hungry early game.

Mid Game
According to how the team is going you go support other lanes if it isn't going well with them. If it is going well, keep pushing until the turret is broken. Go help out your teammates with killing the enemies with help of cresendo if they try to get away. Try to have your powerchord charge set on 2 when using cresendo as it counts as a count letting you immediately use powerchord after your Cresendo doing quite some damage. This usually scares enemies that underestimate you and try to get an 'easy' kill on you.

Late game
Going full support don't mind if you don't have the full build, just stay supportive helping your allies gaining kills, and keeping them alive. Going into teamfights you'll stay a bit behind your allies coming forward if they are close together to hold them with cresendo. Try not to steal too many kills in a teamfight only final hit them with Hymn of Valor if it's the only choice you have and too far away for your team to get a kill on it. If the team splits up go along with the carry.