Sona is a very good support, but is also a harasser. She can support any team battles and makes sure her team mates stays alive. Her skills are enough to keep her team mates alive so there isn't really any need for support items, which means you can get AP items! :D
Sona may seem weak, but with the right items, she can carry! Sona doesn't need a lot to be effective.

Her passive is important to her harassing so take advantage of it.

Early Game:
When the game starts, use clairvoyance at the enemy's base to see where they are going.
Immediately, take Hymn of Valor(Q) as your first skill, and keep using it until your passive is ready.
When you get to your lane, use clairvoyance in the brush to make sure it is safe to go into. If you see an enemy champion in the brush, charge in with your team mate and go for the kill!
Always have your passive ready, harass your enemies, and support your team with heal and speed when necessary.
Heal when you or your team mates are dying.
Speed when you or your team mates are chasing or escaping from a gank.

Mid/Late game:
You should start looking out for your other team mates to see if they need help. Sona is always a great help to everyone whether in team fights or not.
Keep using Song of Celery(E) to help you get to them quicker.
Sona's ultimate(Crescendo) can really turn the game around when your team is losing. Use her ultimate when the time is right(team fights, ganks, etc).

That's pretty much it so REMEMBER to harass your enemies and ALWAYS heal and support your team mates. :)
P.S. Stay behind your team mates. Sheen/Lich Bane + passive = EXCELLENT combo.

Since Sona is a support, you should let your team mate farm while you support him/her. Harass the enemy champions while your team mate farms. If you're team mate decides to go back to your base, stay by the turrent and last hit every minion while you can.