Using runes of force gives you aditional ability power at end phase of game, so your healing will be more powerfull and it allows your team to keep pushing without going back.

I gave masteries to Mender's Faight(defense) and utility build, so my heal have very low cooldown, it helps to survive at begin. If you feel strong you shuld move the point from defense to something more usefull (for example the Insight or archaic knowlage).

First of all you shul buy boots of speed. Becouse of heal and clarity spells you dont need potions so you will have about 100 gold more than your oponents from the begin.

Main item you shuld buy as fast as possible is Tear of the goddess. It gives you lot of mana and some ability power at mid-game;

Some of you probably asks why boots of mobility. Reason is simple: If your team fails push sona must stay behind to use her ulti and stop chasing enemies. This way of protecting team makes sona a good target for exhaust. If sona have +5 speed she willl be able to run away fro such situations. Second reason is that sona need to be always near team when needed, faster sona = more aura.

Third item is soul shroud. Its aura rely helps team, and becouse sona always need to stay er team with her own auras, its good to have aura items. This item also gives sona aditional mana regen and cooldown reduction that she realy need at mid-game. Hp is also a good reason.

All other items shuld be a combination of ability power, colldown reduction and some team aura items, you can choose what you need, i always buy the Deathcap for the 30% AP bonus.

You shuld buy Hymn and aria whenever its possible. Aria use useless until level 8 cose it use too much mana, dont use it when you dont ned it.
So why Aria first? For aura. If your team tries to get first kill at 3v3 defensive aura really helps, otherwise start with Hymn.

Your ultimate is a weak spell so you dont need more than first rank, you can leave it to end, but I always buy it whenever its possible.

You shuld get rank 1 of Song after buying Tear of Goddess just for spaming it. You dont really need this spell at begin and mid-game.

HEAL: It looks stupit to use heal on a healer champion, but sonas healing is almost useless at begin (heals 30hp at rank 1 and use lot of mana). It helps at begin and gives you aditional healing power for critical situations.

CLARITY: Mana, mana and more mana. Sona uses lot of mana from begin, and even more when you buy Tear and start spaming spells. Using clarity whenever its possible at begin and mid-game alows you to stay at lane longer (more exp and money) and you dont need to buy any potions (even more money). That gives you a good boost so you can have as big level on a dual-player lane as on a single-player lane.
At end phase it helps your teem to keep pushing, if you give them mana and heal them whenever its needed you can keep pushing without any recalls for long time