Ahri is my main mid for the longest time. If you need anymore help add me and we can discuss other stuff
You have to land her combo, she in essence is a high risk high reward champ. If you can land charm, pop Fox Fire then Q through them. Its a lot of damage, but you have to get Q to pass through them both times that were a lot of the damage is at since its true damage.
In team fights you want to try and hit a champ out of position with your charm, pretty much be an insta kill or a forced initiation from the other team which your team can capitalize on.
When going into a team fight position is key with you Q try and get it to pass through as many champs as you can, remember both in and out. Then pop Fox Fire and dive in with ulti, its a huge amount of damage. Dive through the team one more time, or hit zhonya's if you are getting focused, then dive back out of the team fight. By that time Q and fox fire should be back up and and lite them up again.