Hi I main Sona and this is the build I have that works for her in every single game. This is what works for me and hope this helps you too. :)

I take these runes because the marks help with basic attack poke and can help secure a kill. The seals are obviously there for more defense. The glyphs increase the damage output.

Since you have a Talisman of Ascension that gives Gp10, you don't need Utility masteries. Mana regen and health regen help a lot in the landing phase. I take Offensive masteries because it helps Sona's poke a lot and can help secure a kill.

These items provide 40% CDR for maximum spamming of abilities and give a good amount of damage and utility.

I take Q first because Sona's damage output at level 1 is pretty strong and you can have kite potential. I max W next because heals are AMAZING. I max E last because it only becomes useful late game. Whenever you can, put a point in R.

Exhaust is one of the best support spells ever. Flash will help you escape.

In team fights, use spell rotation and spam EVERYTHING! Athene's will usually provide enough mana regen for this. Basic attack anyone your team is focusing and stay in the back where your auras provide support your team needs.

Power chord is what makes Sona amazing. In laning phase, using your Q and basic attacking with a power chord immediately afterward deals great burst damage. When attacking a turret, always activate your Q so you do more damage to it. Power chords work on turrets, inhibitors,etc. However, the Q power chord is the best because it deals double damage. The W power chord doesn't affect turrets. Only use this like you would use exhaust. The E power chord is great for chasing someone. Q=damage W=damage debuff E=slow