This is my healer build.

This build takes into consideration 4 things:
1) Sona is slower than most ADC
2) Sona has no AoE
3) Sona has poor mana regeneration early game
4) Sona has a small mana pool late game

This build stacks AP to heal and to do magic damage, while using Cooldown Reduciton to make up for no AoE. Also uses the Alarcaty Enchantment to stay out of harms way early on, as it uses heal and clarity summoners spells (so no flash or ghost for fast get aways).

At level 18, this build will allow Sona to cast an Aria of Perserverance doing a 235 heal every 4 seconds.

With 2075 mana and 65 mana regeneration at level 18, Sona can do heals for a little over 2 minutes before running out of mana.