Hi this is my sona build which won me hundread of games.
First thought you must realize when you play support is dont steal champions kills or minions kills. You allies need them more than you. I hope this build will be useful.

Well let's start with runes. You as Sona need really high health and mana regeneration. Sona's abilities are awesome but there to expasnive when you use them frequently. Next group of runes are cooldown reduction. Sonas heal can reach pretty high amouth of heal and you can use this ability each 5-6 second without cooldown reduction runes. With them and full item set you can heal each 3 seconds. That's awesome, isn't?

Sona need a lot of mana so I chose her Tear of Godness and then Archangel's staff. Then if you and your team need escape you can use Shurelia's Reverie for running. Then you need some amouth of ability power for amouth of healing so thats you have Rabadon's Deathcap and Rylai's Scepter for AP and HP. Boots of Lucidity for cooldown reduction and finaly Abyssal's Scepter for ability power, magic resist and aura effect. This is my item set.

Even though Sona's support, I chose Hymn of Valor at 1st level. 20 heal won't your ally that's why is much more better take DMG on 1st level and then max Aria of Preseverance for healing. Next thing you must realise as support if it needs sacrifice yourself to save your allies or to kill enemies. If enemy is chasing your ally usee Crescendo to stop enemy and let your ally escape. This is very harrash and enemies won love you for this

So. This is it. I'll try to update this build as much as possible and sorry for my bad english.