This is the way I play Janna. People have mocked me for the fact that I go for 2 and then even 3 deathcaps and the reason for this is because a super ability power Janna rapes, nuff said. By the time you get your 2 deathcaps your sheild, which recharges about every 6ish seconds will block around 900 damage, and you can get that just about the time you are reaching level 18, so the game is still usually pretty long from ending. I get more AP quickly because once I get enough then my Howling Gale, after 1 second of charging, can take the entire minion wave, (super farming) which works great for defending your towers because once every minion is dead then the champs are way less likely to tower dive, so, if you're smart you can defend one tower against 5 champs. And if they do dive then you can sheild the tower for about 900 damage, so win win situation.

I get magic pen with the runes because late gate when you have all of your ability power you will find you can output a helluva lot of damgage, to champs and minions. So magic pen is essential. Mana regen is kind of a no brainer but for those with no brains you get mana regen so you can spend more time out on the feild and less time in the base, you're a support character so you need to be able to support, this is important. I get cd runes because the more you can spam your abilities the more you can rape face.

This is the materies build I use, i go more for mana regen instead of increased buffs because sometimes you won't be able to leave guarding the tower in order to get the buff. Oftentimes towers will fall if it is anyone but you left to gaurd them (See summoner spells as to why this is).

As I said before, people often mock me for my item sequence. But, after you start to get AP and start saving their ***es, then they usually stop. I find usually after I get 2 deathcaps then I have enough AP, but as this build suggests, I has more Ability Power so why not get more. It just means more sheild power, and more champ fighting power. I also get 2 deathcaps because even though the effects don't stack, this item is still the one that gives you more AP the fastest. And since this character has a slow/increased movement speed then you don't need to waste gold on more utilitized items. But I stop and get the void staff so I can get more pen to take out those nasty tanks.

I max Howling Gale first so once I get to level 9 I can start one hitting mage minions and start farming. I max zepher next because in its higher levels you get more movement speed and this is extremely essential, if you can't catch or run away then you will end up dead and if you're dead then towers fall, because you work best for defending towers. Zepher is last becasue with you increased ability power it is still extremely effective until you max it out.

This character works very beautifully because by the time you reach level 9 and get a fully upgraded Howling Gale, then you can take out the magic minion with one fully charged wave, and then its all uphill from here. You then proceed for instant AP and can quickly start taking out entire waves. Howling wind also works very well for taking out larger minion waves because as the mages fade out in a semi circle while they attack something. You place your gale so it moves up and takes them all out at once. And in order to make them stay so the gale hits, then you sheild and just sit and take damage for 3 seconds so while the minions wait for game over.

The reason I go for clarity should be obvious. This character needs mana to function, in order to spam your abilities. And I get teleport simply for when you are out of mana and clarity is on cd. You wait until right before the minions engage then you do Howling gale to weaken them all, then quickly recall, resore mana and grab items (as needed) and then teleport back to the tower. If you do this right you can get back to the tower just as the champs are coming into range, you sheild the tower (try and keep towers alive, they are kind of important) and then you do a insta-howling gale (this is when you don't then it charge up, you instantly release it) and then zepher the weaker, or closer champ (your call) and then it usually makes them fall back so your tower doesn't take any damgae at all, works great.

Pros: This character is an extremely effective support. She works great for defending towers and everyone loves how great his is at saving and allied champions butt. She is effective because she has and increased movement speed passive and ability, so she can get really fast. Which means she can catch champs and then use that same ability and slow them. Then proceed to insta-hw and thats a lot of damage, plus knockup so your allies can catch up.
Cons: The shield on this character is extremely effective with this much AP but remember that Janna is still an extremely squishy champion. So if you shield someone/something else, then stay away from the fight because until that sheild is back you are extremely vulverable.

I'll make the summary quick. I know you want to jump in and give the build a try so I'll be brief. Janna is an extremely effective character if you go ap because all of her abilites are greatly affected by it and once you get some good AP on you then you become an extremely effective support character. This is my first build so don't hate on me too bad but definitely suggest areas that need work. I look forward to hearing from you.