If you'd rather play defensively, then it's best to stay behind your carry and make sure you activate the right Auras whenever needed. Specially if the enemies have Blitzcrank or Leona try not to be an easy target for them. It's best to also place a Ward on the brushes to better predict their next move.
If you play aggressively though, you need to be really wise when it comes to your passive, Power Chord and take good advantage of it. For maximun damage, activate Q before hitting an enemy instead of W or E. Harass the enemy AD with Q as well, being carefull not to get out of position and get harassed yourselves. Whichever playstyle suits you best, never forget the importance of good warding for both avoiding ganks and for picking up kills when the enemy least expects it.
Tips & Tricks
Play smart. What does this mean? Just because you’re supporting doesn’t mean you should position yourselves melee-like, or that it’s acceptable to die frequently. Save your ultimate for the right moment when in team fights and never use it for single targets (unless this means you will save an ally or secure a kill, specially early in the game).
Think smart before using utilizing your passive (Power Chord) : If you’re chasing down on an enemy and you wish to slow them down, activate your E before hitting them. If you wish to deal more damage then activate your Q and if you want to reduce the damage dealt by an enemy, activate your W causing 20% reduced damage for 4 seconds when you hit them.
Sona goes well with possibly every AD carry, but does specially well with:
Ezreal (due to the never ending poke from both)
Ashe-Miss Fortune (due to the AOE combos)
Corki (he is a very strong champion with a lot of burst potential alongside Sona’s Q spamming)
Try not to pick Sona if the enemy support too:
Blitzcrank because Sona is too fragile and if she gets hooked in, she will instantly die. However, if you meet an enemy Blitzcrank and you manage to dodge the hooks, try to harass him until his cool downs reset.
Also Sona doesn’t do too well against Leona either, because of her many stuns and Sonas incapability of escape.