Janna brings insane power as a support. She keeps her ADC mobile and her kit is a great set of peeling tools to keep her AD Carry alive. She has a shield that buffs AD and is the ONLY one that can actually defend your own towers. Yes. Even Azir's ;) Her ult allows for some great play and insane heal if channeled fully with good AP. And she has some awesome skins?
Did you know that once you get around 350 AP with her and you ult your team is pretty much invulnerable to all non'burst damage. Your heals even with masteries become strong to that point at around 350 AP. For some more fun try taking runic armor and buying a spirit visage and a full ap Janna healing you. :) Mundo REALLY does go where he pleases. :)

HOWLING GALE(Q)=ACTIVE: Janna summons a whirlwind at her current location which charges up over 3 seconds, increasing its range by 35%, damage and Airborne icon knockup duration each second.
MINIMUM MAGIC DAMAGE: 60 / 85 / 110 / 135 / 160 (+ 35% AP)~This makes Mejais ever the better with such a weak AP ratio. :(
At the end of the duration, or upon reactivation, the whirlwind launches itself in the target direction, dealing magic damage to all enemies it passes through and knocking them up for 0.5 seconds, increased by 0.25 seconds for each second spent charging up to a maximum of 1.25 seconds. And yes. Yasuo's mains galore at the prospect of landing ults with this . ;)

ZEPHYR(W)=PASSIVE: While Zephyr is not on cooldown, Janna is aided by a wind elemental who Ghost ghosts her and grants her bonus movement speed.
MOVEMENT SPEED BONUS: 9 / 11 / 13 / 15 / 17% (+ 2% per 100 AP)(More AP incentive)
ACTIVE: Janna commands her wind elemental(or cloud with Forecast Janna) to attack the target enemy, dealing them magic damage and slowing them for 3 seconds by an amount capped at 80%.
MAGIC DAMAGE: 60 / 115 / 170 / 225 / 280 (+ 50% AP)Note this can be a high nuke with higher AP levels. ;)

EYE OF THE STORM(E)=ACTIVE: Janna shields the target allied champion or herself for 5 seconds, granting her target bonus attack damage while the shield holds.
SHIELD STRENGTH: 80 / 120 / 160 / 200 / 240 (+ 70% AP)
BONUS AD: 14 / 23 / 32 / 41 / 50 (+ 10% AP) AD
Eye of the Storm can be used on turrets, causing Janna to be credited for any kills it earns while the shield holds. The plays I have made with shielding a turret and then flashing and ulting an enemy ADC under the turret and actually killing them in insane. Exhuast also just aids in this.

MONSOON(R)=ACTIVE: Janna knocks back all nearby enemies and channels for 3 seconds, releasing soothing winds that heal nearby allies every half-second for while channeling.
「 HEAL PER SECOND: 100 / 150 / 200 (+ 50% AP) 」Note that as they are displaced Yasuo may ult from the knock back.
Moving or reactivating Monsoon ends its effects prematurely.Avoid trying to have to cancel such a insane heal unless vital to survival. Flash plays with this ult have won whole games for me. Just one flash ult with Janna can decide the whole game. Near Baron pit and alike and the tight spaces this ult can completely ruin the enemy team.

Pros?
Such a great peel support.
Good CC
High synergy potential
Mobile and comes with shields and heals for ADC.
Can be played mid if you're really good.
Rarely dies if played well and it's fun not to die :)

Cons?
Squishy :(
Does not bring tankiness to the team
Rito nerfed her ult heal. :(
Easy to play but very hard to master and timings are hard.

Overall I believe Janna Windforce is an amazing champion. I've maained her since S3 and she always delivers good results. She needs skill but I'm sure with enough time you can become a great Janna.

This is my first guide so please do leave any comments and suggestions for the future. I wanted it to be simple and easy to understand with no fancy artwork or stuff. Simple and right to the point. :) My summoner name is RNX MissUniverse on EUW if you need to contact me for any questions about the guide. I will be happy to help!
"Protect. Ward. Dominate"~The mantle of the supports. :)