Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
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