Helloooooo! This is my first guidee so please go easy :P
This guide is all about Janna! The free air conditioner : )
So why Janna? Well, her large amount of CCs makes her an excellent pick for laning, ganking, escaping, and team fights. I play Janna as full support, to allow my team to reach their full potential.
Supports also ensure the team has sufficient map awareness so dragons and barons aren't easily taken, and can prevent ganks from the enemy team.

THIS GUIDE IS STILL NOT 100% COMPLETE I WILL BE ADDING MORE THINGS ONCE I FIND OUT HOW TO USE FREAKING IMAGESHACK OR WHATEVER LOL. BUT ANYHOW, ENJOY, AND FEEL FREE TO GIVE ME SUGGESTIONS ON HOW TO MAKE THIS GUIDE EVEN BETTER! :)

PROS
- High CC
- High mobility
- Movement speed passive
(which everyone loves ;))
- Shield which gives ad boost

CONS
- Ulti could accidentally blow enemies away >.<
(I mean like, when you're about to get a kill & you flash and begin to ult, but the enemy flashes at the same time and makes you blow them away from you LOL)
- Ulti = high cooldown
- No heal during laning phase :( unlike other supports (eg. Sona, Soroka, Taric)

Recommended

Map awareness is key to winning games. Sometimes your ward in a key spot just faded out and there is no way you can get to the place in time to put up another ward. Clairvoyance provides sight of where you can't reach. Thus it is a must take for all supporters.

When your team is in a poor position in team fights, you can quickly flash and cast Monsoon to reset the team fights. It is also very helpful for escaping.

These are the two summoner spells I usually take, but I sometimes switch out CV for heal.

Sometimes, my AD bot wants me to take heal. Since Janna doesn't have a heal during laning phase, heal is a great option too. (Just remember if you're not taking CV, you have to do a better job at warding.)

I rarely take exhaust, but there should always be at least 1 exhaust on your team. If your teammates don't consider taking it, then you should.

No-go's

I use this for when I'm trolling.. why would you even consider this in the first place LOL. If you actually take this for legit games, I think it's time for you to unistall..

Are you support or jungler? Make up your mind maaaan.

You're a support, unless you wanna piss off your teammates and ks, don't take this LOL.

If you're constantely low on mana, that probably means you're spamming your skills too much.. which you shouldn't be doing! But I guess if you really wanna, you could take it but I would recommend the first 4 SS listed.

Any other summoner spells not listed are spells I haven't tried yet, so I won't say anything about them -3-

Tailwind:

This is Janna's passive, which gives everyone on her team a movement speed increase.(But only while she's alive.) I think this is one of the best passives in the game. :3

Howling Gale:

This is Janna's Q. It launches a tornado in a given direction. You can cast it quickly, or let it charge. The longer it charges, the more damage and distance it gets. This spell is great for disabling an enemy such as Nunu & Willump and Malzahar.

Zephyr:

This is Janna's W. When it's off cooldown, it allows you to avoid collisions & gives you an increase in speed. (You know when you're running away from an enemy or something and those damn minions are in your way =___= but with this, you don't need to worry about it ;))
It can also be used to slow an enemy.

Eye Of The Storm:

This is Janna's E, which is so freaking op <3. This provides an ally with a shield, and also gives them an AD boost. (Well ain't that just dandy?) At level 5, it provides 50 bonus attack damage on who bears the shield and that is equivalent to a B. F. Sword. Even if the shield is broken, it still gives the ad boost. (OP, nerf irelia.) Oh, and did I mention? It can be used on towers too.

Monsoon:

This is Janna's ultimate, and can be used to either push enemies into your allies, push enemies away from your allies, and also heals everyone inside your circle within time. Only con, it is a channeling spell, so it can be interrupted.

START

sight ward sight ward sight ward

I like to start off with a Faerie Charm, 3 sight ward, 2 Health Potion, and 1 Mana Potion.

EARLY GAME

philosopher's stone sight ward sight ward
Then once I reach around lvl 6-7, I like to recall and buy Boots, philosopher's stone, 1 Vision Ward, and 2 sight ward

MID GAME

FOR HEAVY CC ---- FOR WEAK CC
--- ----
sight ward sight ward
The next time I recall, it's usually around lvl 9-11. If the team has heavy CC, I buy Mercury's Treads, but if the enemy has weak CC, then I tend to go for Ionian Boots of Lucidity and top if off with a Oracle's Elixir and 2 sight ward.
Q:So why a Oracle's Elixir at like level 9?
A: Well, because I'm asian and very cheap, I like to take the easy way out. Instead of spending all my money on vision wards, I just invest into a Oracle's Elixir. But it's your choice. I just find this way more convenient, since people don't usually focus supports.

LATE GAME

As the game goes on, you'll want to purchase Aegis of the Legion, Shurelya's Battlesong, and a Locket of the Iron Solari.

Aegis provides you the survivability you'll need, specially the MR. The aura gives +13.5% effective HP to all your team mates, which is really good! But I wouldn't recommend you buy it if the tank on your team already has it!
This item gives you another 15% CDR and an active that increases the movement speed of all allies within the area for 3 seconds. That means your team can quickly run away from a bad team fight or quickly chase up to the fleeing enemy team.
The locket's active synergies with Aegis very well due to the extra resistances and it has a pretty big range so you don't have to get closer to the enemy, and let's you get assists on all the kills. It's a bit expensive, but it compliments Janna lack of heal.


sight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight ward

REMEMBER EVERY TIME YOU RECALL, BUY SIGHT WARD. I CAN NOT STRESS THIS ENOUGH.
WARD ALL THE PLACES!

sight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight wardsight ward


Usually, my game would end by the time I built my locket, but sometimes your games will be a lot longer.

Optional Items:

Banshee's Veil Banshee's is a good pick against a strong AP carry or just a heavy AP team. It offers magic resistance, mana and health and thus everything you need to survive with. Did I mention it's passive? Every 45 seconds you have a shield that will protect one negative spell. This is extremely important in team fights as the first spell usually thrown out is some sort of CC.

Randuin's Omen Take this if the enemy's team main damage hero is their AD Carry. (For e.g. Vayne) The active on this will slow both their attack speed and movement speed as well as providing you with some armour.

And lastly, but not least:

Rabadon's Deathcap Now why this item? Well, if you're not getting hit at all by the enemy team, then defensive items are pretty useless.. But you might as well use that to your advantage. Consider buying a deathcap as it will provide a huge boost to your shield and heal.

Let's get on to warding! Now, because you are playing support, you will be in bot lane, correct? So pay attention to the bottom part of the picture.
During early game, you should be warding somewhere close to the two pink dots. I usually put my ward down around 3:00 minutes. Make sure you CV before you go to ward, because the enemy might be doing a level 2 gank. If the enemy support is constantly camping your brush, it's optional to ward it. (the two green dots.)
After you recall, you should've gotten at least one vision ward, and that should be placed at dragon (pink dot.)
If your team plans on doing dragon, make sure you clear any enemy wards ahead of time.

If your jungler is counter-jungling often, then help them by warding the enemy blue buff (hence red dot).
But, if the enemy jungler is counter-jungling, then you should ward your red buff. (brown dot)
if you're on purple side, then vice-versa.

When the game starts, I like to CV the enemy's base around 0:12, to check their builds/items. (Blue/purple 1, depending on which side you're on.)
Then, 30 seconds before minions spawn, I CV either their blue or red. Make sure you know who their jungler is! (Blue/purple 2, depending on which side you're on.)
for e.g Lee Sin always starts at red, jarvan II always starts at blue.
Remember to always CV Dragon/baron when there are MIAs on the map & your ward was destroyed.

These are common spots that people CV, not the only places you can CV, but a good starting point.