Annie is one of the most viable mid champs out there right now. After her nerf/buff which reduced her stun time but made Tibbers' cooldown scale with level (100/80/60), she is even better in the mid lane. She is best build AP, with a bit of extra health if you like. Start off early by harassing the enemy as much as you can without taking tons of damage. Once she gets tibbers up, you can stun them, hit them, then deploy tibbers for even more stun to finish them off. Have fun playing Annie, and remember to play smart and not overextend with her.

Flash and Ignite are the two best spells, as executing the flash+tibbers combo will do tons of damage to your enemies. If you don't have flash yet, you can also take barrier for even more protection from damage.

In teamfights, her flash+tibbers combo is deadly, and should be used often as the enemy team will be stunned and take a ton of damage at the same time. Then, let your suicidal bear go after them as you and your team clean up.