I play Sona almost exclusively at the moment on Dominion.
Theres a few builds Ive had success with.
I like to start with either Boots/Tear or Boots/Blasting
Boots/Tear is great for building mana by spamming your abilities, Sona can run into mana issues as your survivability and ability to kite can mean you stick around for longer. Tear makes this a null issue. I then build Ionian, Sheen, then from there you can either max your CDR for late game 60sec ultimate to control team fights and points, or finish your tear if you have the gold.
Boots/Wand goes into Ionian then Morellos for max CDR, then sheen/rush Deathcap, then survivability items etc.
Tear gives you the sustain you need to spamming continuously and when you finally upgrade it, you get some decent AP with your other items. Combined with sheen, and powerchord this allows you to push decent DPS. While the CDR early push (consider frozen heart over morellos depending on enemy team comp) is also viable, getting your chords down to 4.2 seconds each and your ult to 60 early on.
For masteries I either build a 21/0/10 or a 10/0/21 depending on what im building. Both work. You can have games where you go 6/6/28 or 12/2/18. it varies depending on your team comp and how good your team is.