First let's talk about my philosophy of a support and his role in Summoner's Rift.
The support role was a very underestimated and mistreated role before S4, often it was considered a burden to the team and it only served as simply a champion who buys wards for the team.
Since S4 came out and the support role was enhanced significantly, players started to notice it's values and utility to the team.
The support is someone who is there to help you get stronger, he gives up his greed for kills and gold and allows his teammates to get the goodies, he also gives up his desire to be in a spotlight by helping his allies make great plays and enouraging them to get better, keeps the morale of the team high when having problems and comforts when mistakes are done.
All of these makes the support an extremely team-dependent role.

Pros: +Good early, mid and late game +One of the best knock-up spell in the game +High mobility +The best shield in the game, period +Gamechanging CC +Best disengage

Cons: -Little damage-dealing capabilities -Extremely team dependent, to a point where you become useless if your team sucks -Hard to master

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First let's talk about Janna's abilities and how to use them:

Q Howling Gale : This is the spell that in my opinion makes Janna the best support in the game. It's very versatile and unpredictable sometimes, it can be used to initiate teamfights, disengage, avoid ganks, plan escapes, interrupt channels and can interrupt most gap-closers in the game, just think about all the possibilities.The more this ability is charged, the nastier the knock-up. The key thing about this ability is to THINK AHEAD, the more info you can get from the zone you are into ( that means having the zone warded ), the more you can think ahead and more precisely. There are other similar spells in the game similar to this that dismantle enemy champions in a zone, but with this baby you can knock-up the whole enemy team if you catch them in A LINE.

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W Zephyr : This ability compliments more of Janna's mobility and CC. As long as you don't use it, you get more movement speed and you can move through units, which is a nice thing to have. Just use it when you need to slow somebody. Things to keep in mind : in a teamfight slow the enemy champion who focuses your carry ( it will most likely be melee ), and it's also useful when chasing.

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E Eye of the Storm : This is why carries love Janna, not only it gives your target a shield but it also boosts it's damage, and to make it even better, later on it can also give an attack speed buff. In laning phase keep in mind to use it when you predict a trade or depending on who you are laning with use it to harass. It can also be used for some clutch saves that will leave your enemies like : "omgwut? my q did no dmg to you, gg rito...". Keep in mind that it can also be used on towers.

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R Monsoon : Now this is where Janna's ability to change the game completely comes into play, use it good in a teamfight and it's guaranteed win, use it badly and it will probably cost you not only the teamfight but the whole game.