In my opinion Janna is the best support in the game, due to her mass utility and ability to baby sit carries or anyone else that has troubles last hitting in a lane.

Janna can make or break a team fight, if played correctly she can easily turn a team fight into your favor however she can also break a team fight if played incorrectly.

However in this guide I will be focusing more on roaming, since the roaming meta game is really broken atm.

Janna is a very versatile champion, she can babysit very well but can also roam very well.

Right now the roaming meta game is really strong and having a roamer and a jungle is very strong, especially if you have someone like Cho'gath or signed holding bottom lane.

When roaming with Janna you always want to harass the solo lanes as much as possible, that means top and mid. You want to open up with, auto attack twice,and auto attack, this will maximize your damage when roaming around.

Harrassing
Roaming takes a lot of patience, you want to basically camp the top bush and make sure don't get spotted or seen by the enemies and as soon as they get aggressive or push the lane you want to punish them by the combo that is shown above, then you can pretend to go mid lane and come back around through the top push and sit in there again and begin harassing, what this will do is it will make them play more safer which means they lose CS and you can also deny them XP giving your solo XP advantage. You can also do the same to mid lane.
You may even get them to blow the summoners spells which will leave them vulnerable gank them when summoners are down.

Not only can you mess around with the lanes, you can also screw around with the jungler harass him when hes taking buffs and making him B will slow him down a lot and who knows you might even catch him low and net yourself a kill.

To make roaming more effective gank with your jungler to get the kill for sure.

Wards
When you get your Oracle you want to roam around clearing wards to remove their map awareness

Why Roam?

Roaming gives you 3 solo lanes, and if you can hold the lanes this will give you a big advantage in the late game stages, and harassing the lanes and the jungle will give your lanes a bigger CS advantage which means your lanes will be more farmed, and since Janna doesn't need to be farmed she is a viable roamer.

If you want to learn more about roaming watch this quick video I made on how to roam with Janna effectively

http://www.youtube.com/watch?v=MycHkrNU5-o

Not only is Janna a great roamer, she is excellent in lane

Janna has the ability to let a carry or someone who can't farm, be able to farm.
However babysitting in lane can be very boring as you basically will be sitting there making sure your lane partner doesn't get killed.

When laning you want to play aggressive and passively,just use your brain and you'll know when to play aggressive or passively, you also don't want to take any CS let your partner have all the CS only CS when your partner needs to B or if you know hes going to miss the creep

Harassing
Janna can be extremely annoying when going against her in lane, what you wanna do is put on your shield run up to the enemy spam your spells and auto attack, the back off and when your CD's are over do it again, be careful though the enemy may get annoyed and request a gank from their jungle.

Marks : Armour Pen
Seals : Flat Amour or Mana Regen P/ Level
Glyphs: Flat Mr or CDR
Quints: Movement Quints Or Healths Quints

Why do I choose these runes?

I choose Armour pen runes because early game you wanna maximize your damage output when roaming, Armour pen runes let you do true damage to most champions when auto attacks, usually you can get a few auto attacks in between your slow and knock up, which will improve your damage when roaming, Flat Armour helps mimize damage you take from ad characters when roaming, however you shouldn't really taking damage when roaming but if you do flat Armour will help, you could also use Mana regen however when roaming you don't need to spam your skills as much, however if you decide to baby sit and roam I would use mana regen so you can spam your skills a lot more, I take flat MR to minimize magic damage early game, CDR runes are only for baby sitting because you wanna spam your skills to harass or protect your teammates, Movement quints are for roaming so you can close distance and stay on whoever your trying to harass, healths quints are also a good choice since Janna has good base movement speed anyways.

I go 0/9/21 For masteries

Janna has good utility and going 21 in utility masteries will give her a even bigger utility boost.

I go for the armour and mr in defence tree just to take less damage when roaming and strength of spirit mastery for a little health regen when roaming, if you do end of taking damage.

Starting Items :
Boots , Ward, HP pot

When roaming boots give you a big advantage cause most people start off with dorans items so you can close distance and be able to harass a lot better, Ward is for warding the jungle so you can gank the jungler or just get more map awareness.

After that you want to buy oracle to clear wards and get rid of all their wards and remove their map awareness so you and your jungler can gank easier, it will also get them playing a lot safer which means they lose CS, however if don't decide to play safe you can take advantage of this by ganking them.
I buy 2 Philosophers stones to get a gold, since your not going to be last hitting anyways other champions need to be farmed while Janna doesn't.

The new update allows you to change both philosophers stones into an item now and since Eleisa's Miracle now grants tenacity you can buy this and have cc reduction and buy different boots instead of merc.
An important item on Janna, Will give a nice boosts to your teamates, this is usually the first or second items that I buy as Janna
Good Item to stop those casters from bursting you down, buy this if they have a lot of magic damage
You usually don't get to buy more than 2 items as Janna but if the game drags on and you need more HP, this is a good items, it also gives some nice CDR.
An okay item can help you close distance or can help you and your teammates survive a gank, gives some CDR and more health and instead of selling both philosophers late game you may want to change one of them into this.

Other good choices
I usually don't buy this but if your having troubles and need a bit of extra damage buy this and make sure to use the unique active, it also gives a fair amount of ap, mana regen and CDR.
If your doing really well you may want to just buy items to help up your damage input, and this is the perfect item for it ;D.

Wards

It is always important to be warding as Janna, buy wards when ever you can to give you more map awareness, buy pink wards for drake and baron, or for any stealth champions. Wards can change a gank into your favor.

Good spell for level 1 team fight, usually get this spell aoe knockup OP, Mid to late game useful for pushing and keeping the enemies off your carry, MAKE SURE TO USE THIS TO STOP CHANNEL'D ULTS.

Annoying *** slow and gives you more movement speed, the slow is useful for chasing enemies, open with this when roaming auto attack twice then use Q and auto attack for maximum DPS.

This is your main ability for harassing when roaming with Janna, throw it on the enemy get 2 auto attacks off them Q, auto attack.
IMO one of the best supports spells in the game, shield + BF Sword = OP, Make sure to use this on carries on team fights, it is also useful for saving people that are ignite or if karth is going to ult or if anyone is going to die just shield them and it might just save their lives.
Janna's make or break spell, if used correctly it can really pay off, however using it incorrectly will cause you to lose team fights. To use this spell correctly you want to look at the situation that your in, if your team is winning a team fight and you see someone on your team is about to die and you ult right in the middle, your gonna screw the fight over and reset the team fight, however you can move back a little and not get the knock back on the enemies and heal your teammates OR you can flash in and ult push all the carries back into your team while healing, however try to wait for most of the CC to go down before using. MAKE SURE NOT TO PUSH ANY TANKS INTO YOUR TEAM THAT CAN AOE STUN, this will really cause you the fight.

This spell can also be used defensively, if your getting chased you can ult to knock back, use your Q and run.

Best Choices

In my opinion this is a must on Janna, with flash you can flash into and catch carries out of position and knock them back, also can be used defensively to escape ganks.
This is a must in ranked play with Janna, CV can help prevent ganks when there are MIA's, or can tell you where the jungler is so your jungler can counter jungle , etc


Other Choices
These summoners spells are okay to have but i really don't recommend getting them unless your just pub stomping and having fun.
Only use ignite in non ranked games, you really don't need to be putting out damage unless your doing extremely well, however in normal games this can really help when roaming around.
Good summoner when they have a lot of AD damage champions,
I would only suggest this if they have someone like mummy or Galio so you can reset a team fight into your favor.

Bad Choices
Not recommended at all

I see a lot of Janna's getting clarity, tbh clarity is really not needed because you shouldn't have mana problems in the first place, however if you do you should get mana regen per level seals and you'll be fine.
No point of revive unless your karth, panth ,tf
or if your trolling
Don't really have to go over the rest.

Pros/Cons - Laning Janna

Pros

- Great Baby sitter, can get your carry farmed
- Good harassing ability
- Can escape ganks easily

Cons

- Can be very boring, watching the carry farm half of the time


Pros/Cons - Roaming Janna

Pros

- 3 Solo Lanes
- Can Harass other solo lanes, keep them under farmed
- Can clear wards more effectively, remove their map awareness
- Can Harass their jungler, and slow them down
- Fun

Cons

- If done wrong , will cost you the game
- If bot lane is bad , will cost you the game
- Prone to ganks when roaming around their Jungler

Janna when played correctly will win you games, know when to roam and when to lane and you'll be fine, know when a team fight is lost or won and don't mess up things with your ult always use it to your advantage.

Hope you like the guide :D