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.
Early game Ignite is essential for you and your team to get kills, especially if you are laning with someone who isn't a DPS. Therefore I choose Ignite.
As second ability I choose Flash cause' once the other team has spotted how weak and fragile Janna can be you have to have an escape route. But of course you can choose any other spell you want, this is just what I prefer.
Skill Rank Order
The first skill I usually pick is Eye Of The Storm if I am playing with a melee champion, otherwise I go for Howling Gale first. Be sure to always be faster than your opponent, therefore adjust Zephyr after your needs.
I don't think I have to explain the masterieses since it's pretty obvious why I have chosen like I have done to someone who has played Janna before. The only thing to point is that you can change whether you want to spec Flash or Ignite.
Go with magic penetration, cooldown reduction and mana regen.
Item build and Strategy
First you buy Doran's Ring, for your health, mana regen and ability power, and then a Health Potion. This will let the other team have a harder time to kill you if they ever tries to. And don't waste your mana if not necessary, otherwise you will have to call back, and you of course don't want do that. Just put shield once in a while and use Howling Gale at every minion row when there is the bigger minion with (can't remember the name). Janna almost always dominate lanes. And do always have Zephyr ready before fight.
Then you can either buy Sorcerer's Shoes or Boots of Mobility. Magic penetration is always good, and extra speed will help your escapes or when you chase an enemy (use storm and slow and your team will have a greater chance for a kill). So it is up to what you prefer and how the other team looks like.
If you took Boots of Mobility you should take Haunting Guise for the magic penetration. If you took Sorcerer's Shoes take instead Fiendish Codex.
When you have earned some more money, buy Banshee's Veil, it will hopefully and with big chance save you from any team ganks since with Flash and your great movement speed let get out of sight fast. If they should get you still, and you are stunned - it's good to have Cleanse so that you can use your ulti or Howling Gale to get away.
Then buy Archangel's Staff for mana reg, ability power and so on. If you haven't won by yet, buy also Void Staff which give you greater magic penetration and ability power.
Good Luck and Have Fun with this build! I'm sure it will help your gaming further.