Hmm, I'm usually playing Sona passive/aggressive, depending on the matchup, but since Sona is ap scaling support, I'd recommend to get flat AP marks, flat armor seals, flat/scaling mana regen glyphs and flat AP quints.
with this you'll have enough AP at early game to chunk off the enemies ap and having as well some sustain.
for masteries, I'd recommend 9/0/21, taking cooldown reduction and some AP in offensive, in utility you'll aim at the mana regen and gold income masteries, rushing chalice and other important items. I'd also recommend to get the mastery expanded mind, giving you more mana at the beginning of the game.
http://i.gyazo.com/bc26a13c70792c45c121068c3ff35818.png
you can either put a point into scout or phasewalker, depending on what you'll prefer, to be faster at base or having that increased range on trinkets.
you can also instead put points into the movementspeed ones, I don't think it's needed but if you want to.