Stay with the team. Sona makes an excellent healer, Crescendo is best used when the whole team can take advantage of it, and Song of Celerity lets the entire team kite. Without a teammate around, Sona is little more than an easy gank.
Early game, be wary of constant spell rotation, as it will quickly drain her mana. However, this can also be a large problem mid to late game without proper itemage.
Crescendo coupled with allies' channeled abilities can devastate the opposing team, as while stunned, they can't interrupt the channel.
With her Song of Celerity and a Shurelya's Reverie, Sona can make it nearly impossible for the enemy to catch a dying teammate or escape from her team during a chase. These two things also allow her team to traverse great distances very quickly, such as quickly running to a tower/inhibitor after an ace to take it down before the enemy team revives or moving to a friendly tower/inhib to prevent it from being taken.
Being a support, Sona should focus on supporting her AD carry early game by warding and keeping him/her safe. If your carry can get last hits, you'll want to leave them for him/her. You can offset your lack of farm by buying Philosopher's Stone and Heart of Gold to provide much needed regen, survivability, and Gp10.
A valuable build for Sona is stacking auras, as all it takes is Sona's presence for her to be useful and most of the aura items you should be getting are relatively cheap.
Another useful aura to get is Soul Shroud, which has a great aura on top of extra HP.
Sona is extremely valuable in group pushes, but sustained aura rotation can deplete her mana pool quickly if not equipped properly.
Because Sona has generally low cooldowns, low mana costs, and needs to constantly switch abilities, she charges up Archangel's Staff faster than almost any other champion. This also helps her, as it provides the most mana regen in the game and allows her to stay in long team pushes for much longer.
Mejai's Soulstealer, while not always viable, generally charges up quickly as long as you stay safe. As long as she has an aura active, Sona gets assists for simply being at teamfights.