In-lane poke is very strong, and once you get 6 you need to coordinate excellent ganks and setups. Wait for their ADC to be very far away from their support (1000 units or more) and then ult him, Q to give boost, and use your W proc for the slow. Then switch back to Q for more damage as you chase during the slow.
Once laning phase is over, wait for the other team to line up and if you can ult 3 of them at once it's game over.
Sona's pretty weak pre-6 and she's a one-trick pony post-6, so if you miss Crescendo it's pretty hard to carry as Sona. However, if you land the right Crescendos she's a game changer, which is why she's so popular in LCS.
EDIT: Just to point out, if you DO get support, don't pick Sona. She "can" carry but there are a lot of better supports right now. Tank supports are becoming really strong atm, such as Volibear, Thresh, Taric, and Leona.