start with doran ring and always try to give last hit with your basic attack/Q to get a good amount of minions

after suffering through farming minions with no mana, return to base with 650g and buy tear of the goddess and faerie charm.

after that, buy moonstone and in case you already destroyed the tower or your tower was destroyed, chase who is stronger in your team and focus on healing him and giving buffs to him too. sona has an easy time winning any game if you leave her strong in the early game, so don't worry about it.

in lane:
- always stay under the tower when possible if you have no pressure on the lane
- prioritize buying mercury's treads if you are against mages or champions with a lot of cc
- good luck csing and surviving

in teamfight:

- press W
- stay behind your carry so you don't expose yourself so much, if you are not worth gold, you can die for him
- abuse your E skill a lot to get to teamfights and objectives and rotate to help your allies