You can go and lane middle, you are perfect for there. But you need a lot of experience if you want to, Janna is not so easy to play. Your most important skill in the beginning is Eye Of The Storm which at level 5 you should have 2 points in it. Also You need to play a little bit aggressive... if they gang you a lot, you can escape with flash, with Howling Gate or with your Ultimate..so thats not a problem. These are my runes which i selected, but you can go another ones - but also connected with Cool-down and Ability Power. About the items i chose, i start with Boots and 2 Health Pots. After that, its your choice, but you need to have Mejai Soulstealer (great damage - if you are skillfull and can kill a lot) , Rylai Scepter (perfect chasing item) and Archangel Staff (no mana ending) - most important items for AP Janna ! I think thats enough - if you have any questions - feel free to ask.
P.S: Sorry for my bad English, it is not my main language after all :D