A Short Janna explication:
Janna have a safe lane, she's good with most of Ad carrys, don't have a good pell, but can desabilite all AD fighters melee, and she can save the carriers hard.

Pros:

+ Safe laning phase

+ High disengage

+ Protect Adcarry

+ High MS

Cons:

- Need ult for sustain

- Weak against AP assassins

That Runes make Janna be Safe on lane and give some heal for her be more safe on late game, if u are not against a support with a stronk magic damage i recommend put 6 CDR Runes on blue's.

Level 1 - You just have your E try to push lane and take lvl 2 first, Always when your Adcarry try to trade auto or will receive an auto try to shield him.

Level 2 - When u have Q lvl 2 u CAN'T use him for poke or do DMG, NEVER, u need use your Q for a disengage. If You Get Level 2 First Say to your Adcarry to abuse a little bit, give E for your Adcarry for give more damage for him and get the enemy a little bit scary for fight, If They Get Level 2 First Stay a little behind of your carry and be prepared for shield him if they hurt him

Level 4 - If they engage on your Adcarry. and the Support is melee like Leona or Alistar u should use your W on the support to him don't use more CC on your Adcarry, if he is ranged is better use your W on his Adcarry but don't compromise your position for this

Level 6 - Your ult have like 130 CD in lvl 6, use only if you or your Adcarry will die, u can flash ult if u are badly positioned, but only use for keep your Adcarry Alive.

First item for Janna it's so EZ for think the Ancient Coin is the better item, now i will show why, first I will tell about Relic Shield that item will give life who is good for janna, but is realy realy realy to hard for catch the last hit and isn't worth do dat item it don't have synergy with janna, and is the worst support item for give money for the support, now we discard the possibility for use that item as the supp item now we will talk about Spellthief's Edge that item will give 5 ap, first will talk about it, 5 ap is like nothing for a item, just have more 5 ap will give more 1 or 2 life on shield and 2 or 3 life on damage skills, so that part just show how useless that item are in the lane, now for show more that item just is good when you are a poke support and need

Janna isn't god in do roaming, but she can go on Enemy jungle put some wards, with someone always (like the Mid or Jungler, with only the Adcarry isn't to recomendable) and maybe u can the enemy Jungler, or someone walking there and kill him, or make he use a summoner spell.

With Janna (1-10):

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(+1)
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(+3)
(+5)

Against Janna (1-10):

(-6)
Jinx have do the same thing for counter Janna like Caitlyn, Kog and Twitch, Just poke poke poke with the range with his Q. But like Twitch and Kog' Maw she have the same problem don't have scape, so if u engage her with jungle probably she will die
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(-7)
Caitlyn early range counter Janna because Janna become a duck in a river just waiting for being poked by Caitlyn auto auto and auto, because this Janna have to back base too much turning on one of the almost boring lane ever for Janna. For counter her is just try to stay behind the minions, and shield your Ad or You(shield who will receive the auto) when Caitlyn try to hit, don't fight, and wait for gank to give some kills for your Adcarry.
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(-9)
Graves can burst Janna easily if she's stuned or bad positioned, in early game graves is dangerous because he have a massive area damage
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(-6)
Kog'Maw have do the same thing like Caitlyn and Twitch, Just poke poke poke with the range with his W. For counter him have two ways, first if you are losing trades is just try to stay behind the minions ,don't fight, and shield your Ad or you when Kog'Maw try to hit, If you think you 2 will won the Fight try to beit him and focus on Kog, he have no scape and because this is too easy kill him just engage and use your tornato and will be easy kill him.
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(-5)
Twitch start be dangerous when he's on lvl 6, ulted Twitch have the biggest range ever, who is biggest than Janna's ult range and because this desingage him is hard for Janna if the Support help twitch Janna become a EZ kill for them. before 6 probably you will win so u can try for fight if you think it's safe and you 2 will not die Wait for gank to give some kills for your Adcarry.
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In Lane If u make enemy's Botlane back to base or die, and u are with a good health for make dragon, call jungler put vision on enemy jungle for don't be suprised if his team came for try stole the dragon, and make dragon if possible, is necessary have a pink ward on dragon if don't have someone dead in enemy's team, and u need control vision in their jungle and control dragons.
In Mid Game (17/25 mins) - If your team are winning and starting the Team Fight you need have too much ward deep in their jungle, call your Jungler for help you to make this, and never ever do this alone.
If your team are losing and/or being engaged you need put wards on your jungle entrances and put too much pinks on the brushs for don't let them walk free on your jungle and kill your team, always when you are losing you need control vision on dragon because control dragon can change the game in late making you have a EZ win.
In Team Fights - Try to protect your Adcarry or your Midlaner, if your ADcarry is weak or your Midlaner is more precious in team fight you can focus in protect the Midlaner, you always need stay behind and protect they with your ult, shields, and items, if they are not focusing your's carrys and only are peeling your frontliners try to shield'em and use items who give ms for help them u can ult for heal if is necessary, and if u can do the "insec" if you will not enter in a dangerous positionament

I'm apologize if i have a bad English because i'm Brazilian and here is 4:40 am and i'm too tired, i tried my best, Hope u liked my Janna Guide, and thats all folks