With good AP ratios on her skills, Janna can be considered an AP Caster. However, people rage when they see a Janna mid. With this guide, you can be a support -- and deal tons of damage at the same time!~ (focused more on the damaging)

Janna is known as a defensive support -- all of her skills are centered around her survival. However, she is also a great roamer, and can change the outcomes of teamfights with her skills. This guide acknowledges both of those points, and introduces an element of aggressive play -- this guide will Janna a well-rounded champion (excluding tanking).

Just remember -- YOU ARE STILL A SUPPORT. Give your adc their cs (and your kills, sadly). Be a team player.

flash/exhaust
could substitute exhaust for teleport, heal, cv, or ghost (roaming support ftw, especially a painful one)

gp10 seals/quints: snowball potential, can also help you catch up when you lose early game

magic pen marks: makes q, w, and lich bane hurt more. you could also use armor marks: at .91 armor each rune, they are efficient enough for bot lane.

flat ap glyphs: make your skills hurt more, as well as shield your adc for more damage

Offense: 21
Sorcery, Blast, Arcane Knowledge, Havoc, Mental Force, Spellsword, Executioner -- they are the standard apc masteries in the offense tree
Why only 2 points in Archmage? Because I like to dominate my opponent early in the game, so I move the other 2 points into Brute Force. Basic Attacks still do a lot of damage.

Utility: 9
Summoner's Insight for Flash cd
Meditation -- just my recommendation, but Wanderer and Improved Recall are good too
2 Points in Mastermind and 2 points in Expanded Mind. Reallocate a point to whichever you like more, or keep them even.
Greed -- I just take it 'cuz I'm greedy; put this point in Biscuiteer if you feel it's better for you.

I usually start with Q or E. Do not get w first or second; Q and E are essential for survival.
What to max first? Well, if your adc is good at his role, as in, he can cs and harrass like a boss, I suggest you max E first. However, if your adc sucks, as in, he only had 3 cs after 2 minion waves and took a bunch of harrass from the enemy adc/supp, then max your Q, as you have to carry the game now. It is a good harass/farming ability AT THE SAME TIME. BUT! if you find that everyone on bot is doing averagely well, I suggest you spread your points between E and W, and max both simultaneously; act as if Q were just a knockup without damage, and use W as a good 48% slow for 3 seconds, and E as the best shield your adc can have -- the shield also gives bonus attack damage while it remains.

Why 1 point in ult and max it last? Chances are, you're only going to use your ult for the knockback, cancel ult, and continue running. The ult isn't ideal for healing because of its long cooldown.

Q - if your adc is good, avoid hitting minions when trying to harrass the enemy adc and support. you could also use a quick launch to get minions to last-hittable health, which is ideal for pushing lanes with your adc. pushing lanes into towers is a good tactic, as it makes it harder for the enemy adc to cs, but it could leave you vulnerable to ganks. I suggest pushing waves without the siege minions (the big ones). Your Q is also your main damage ability. Why do I care about damage on a support? THIS IS A HYBRID SUPPORT/APC, FOR AGGRESSIVE GAMEPLAY! Dominate bot lane and both you and your adc will carry hard!
How to use your Q: memorize skillshot width, base distance travelled at 0s charge. when using it for defense, as in, you're running away, cast Q in the direction you are running. this allows for q to charge more, thus providing more knockup. for offense, the q requires prediction of enemy movements, as it is very easy to dodge

W - epic slow at max rank, yet very weak in terms of damage -- use it to chase enemies. Keep the passive (don't use the skill) if you are chased by 2+ enemies. tip: minion waves can sometimes help detour the enemies, as you can walk through minions while you still have the passive.

E - use this when your adc starts to engage an enemy or when a teammate is low on health. Endgame, with all of your ap from items, this shield should block A TON of damage. on top of that, it grants bones AD to the shielded person while it exists.

R - very long cooldown. use it as a last resort escape skill by casting and immediately cancelling to resume fleeing, or use it to heal. Be intuitive: if you are in the enemy's bot brush, and your adc or you are low on health, yet the enemy adc and support are dead, but the enemy jungle is not on the map (headed your way), you could cast your ult in the bush, make sure the enemy jungler doesn't see you heal up, watch him come into the brush, and counter-gank him with you and your adc at a very high health, ensuring a favorable outcome. keep an eye on mid too.

Janna is a defensive support, naturally. However, you are weak, since there are no MR or Armor runes, or defense masteries, or even items, so try to avoid taking damage at all times. Use your skills to improve survivability. However, do not create the mindset of survivability just yet: this Janna that you are reading about is a hybrid: she does some good damage, with good ap ratios on her skills, and should be played aggressively to deny your opposing team's bot lane a chance at carrying the game.

Items will help you be aggressive.

First, I usually get the gp10 items: Philosopher's Stone and Kage's Lucky Pick. They will help you snowball, as well as catch up if you lost earlygame.

Get Chalice of Harmony if you're in for lane sustain, or Sheen for more aggressiveness. Get both if you can afford it.

Build Mejai's Soulstealer if you really want to hurt your enemies. (build it only if you're winning lane)

Always get Morellonomicon -- it nerfs enemy heals (even your basic attacks will apply the debuff, as you have the spellsword mastery), it gives you awesome cdr, makes your skills more op and gives you mana regen -- ideal for roaming

Mikael's Crucible -- another defensive skill in your set, and can make your allies very happy to be alive. Also gives EPIC MANA REGEN FOR THE SPAMMING OF SKILLS (get max cdr asap with this item)

magic pen boots vs cdr boots -- magic pen if you are winning (and if you have soulstealer), cdr if you are losing

rabadon's deathcap for... killing efficiency

lich bane -- op as hell, see the chapter dedicated for it below

proc is available every 2 seconds when you're spamming your skills.

How to get the most of it:
Q -> basic attack -> Q to release 2 SECONDS AFTER Q WAS STARTED (to re-proc lich bane) ->basic attack -> W (again, 2 seconds after Q release) -> basic attack -> alt+E (after 2 seconds...) -> basic attack

shorter and more readable: Q a Q a W a E a

why alt+E? it's self-cast by default. E gives bonus AD as well. if you are able to acknowledge teammates in that 2-second timeframe of a combo, then go ahead and use it on one of them

other skill combos:

E a W a Q a Q a -- soft engage, do tons of damage as the fight progresses

Q a Q a E a W a -- remember, you have exhaust

E a Q a Q a W a -- exhaust helps land Q

W a Q a Q a E a Flash+R a W a Q a Q a Exhaust+E a -- "You're not getting away!"

I suppose I should tell you this: I rarely play normals/have never played ranked. Yes, I play mostly co-op vs ai. I am probably more worse at warding than you are.

This is just a guide; these tips are here to give you insight upon other's playstyles, but you ultimately decide how you play.
This is also a build for fun, as I just use this in co-op vs ai, but can be lethal if you're on a smurf climbing ranked.