Well the good news is you're in bronze, so for Sona it's easy. At that level, build full AP.
for core build. After that, it depends on the opposition. A lot of people are going to be telling you to be running mana items, but i've yet to honestly need it. It's about resource management, not spamming skills like a cat pawing a keyboard.
Sona at your level is a lane bully, so use her early damage to it's full extent. You want to be running Thunderlord's. Sona can chunk down any adc she gets near and a great deal of the supports, even a lot of tanks early on.
You honestly only need roughly 20+ cs if your kill participation is high enough. It goes against what they drill into young supports heads, but the more kills the better. End of the day, any well fed member of the team becomes a force to be reckoned with and at a bronze level Sona can devastate the enemy. You want 65% or high for kill participation. If you aren't getting many kills, you do want as many assists as you can get, with as little deaths as possible.
Besides that, you need to be a warding machine. By 30 minutes you want 20+ wards out, not including pinks. You also want to be trading out your yellow trinket for a sweeper and clearing as many wards as possible.
Not everyone will agree with my advice, but it is how I easily carried myself to a mastery 7 on Sona. You can do it the harder way building more traditionally if you feel like it.