Here's how I build Sona and basically any support character as well (Janna/Soraka/Taric):
Runes: Mpen, MP5/level, flat CDR blues, flat HP or gold/per10 or movespeed quints
Start with two faerie charms, a ward, and a health pot
First back, build 1 faerie charm into a Phiolsopher's stone and buy boots of speed and always buy more wards
Next upgrade 2nd charm into a 2nd philosopher's stone, against always buy wards when you go back
Next buy a Heart of gold and upgrade to 15% CDR boots, buy more wards
Next build aegis of the legion, buy more wards
Next build soul shroud, selling one of your philosopher's stones, buy more wards
Hopefully game is over at this point, if it keeps going upgrade your last philosopher's stone to the new tenacity item, keep buying lots of wards, maybe buy a large rod and turn it into a deathcap if the game goes super long but hopefully that doesn't happen
At some point during all of that, buy a oracles and start killing wards, but only if you are confident that you won't die for quite a while
For skills, start with 1 in Q, then max W first, taking 1 in E at lvl 4, then max Q after W and then E last. Lvl the ult every time you can of course.
TL;DR buy at least 2, preferably 3 gold per 10 seconds items. They are amazing on supports because you want to leave your CS farm to your laning partner and they let you keep up, plus they give you sustainability/survivability