Sona is one of those versatile champions. She can be played so many different ways. She was the champion that made me start playing League.
When it comes to playing Sona there are a couple of things to know.

Sona's passive empowers your basic attack after you cast 3 spells.
Your Empowered Q is a damage buff which scales with ap.
Your Empowered W is a damage reduction for the enemy it was used on - similar to exhaust.
Your Empowered E is a slow for the enemy it was used on.
You have to learn to utilise this passive at its fullest because it might turn the fight around.
Keep in mind when you hit a minion the buff is applied to that minion and you will have to stack it again, same goes for Illaoi's tentacles, Zyra's plants, and Heimerdinger's machines. Killing a ward won't have the same effect though!

Sona's ult is a really powerful one simply because you can stun a while enemy team with it. Keep in mind you don't always have to use it on a couple of enemies. Stunning the one who is fed or a person who is trying to kill you will also put you on an advantage.
The ult doesn't stun minions! So don't try to use it to farm :D

Remember, every game requires different items. Don't always go for the same build over and over again. Imagine having a build that is full of mr and the enemy team is full ad.

In early game you want to be aggressive if your lane allows that. If not be aggressive by punishing the enemy when they make a mistake.
In team fights you need to fight around your team no matter how bad they are. If you go alone you might die.
Your power-strike is late game so even if you are behind, don't give up!

I am aware that Green Sona exceeds the cdr limit and the Red Sona doesn't reach it, that is why I have a rune page that has Transcendence as an option - which allows you to exceed the limit and I also have another rune page which adds additional cdr so the cdr limit is reached.