AP Janna Top Lane / Mid Lane Guide - LONG *TL;DR included at beginning*
Quick version : Max W first, Q Second, R at lv 6, then skip until lv 13 to max out Q Asap. Summoner Spells : Ignite / Flash
Basic Item build : Start with either Dorans or Spellthief Edge + pots, then either second doran's, or complete Frostfang, then immediately rush Luden's Echo. Continue to focus large amounts of AP on items - more AP = more survival. From here, consider either Deathcap for maximizing Monsoon / Eye of the Storm bonuses for late game, or Liandry's Torment to boost your poke damage from W and Q. It is important that you gain the early advantage in lane as Janna, so you need to play aggressive, but her kit makes this quite easy.
So Janna top lane.. WHY?? This is what I get asked every game I pick it. Janna fills a role similar to Teemo and Lulu in top lane - a mobile poke champ with a potentially extremely powerful late game -- just not in the traditional sense. She is extremely fun to play, especially in a solo lane, and is a very underestimated Mage champion - she actually does quite a bit of damage at all stages of the game and shifts quite nicely into end game when a lot of more traditional damage-oriented champions would taper in power.
She has a very powerful presence in lane, and with her extreme mobility and disables, she is impossible to retaliate against for many top lane champs, leading to a frustrating experience for your enemy where they can end up very gold/xp starved. As an extra bonus, almost everyone you play against will underestimate you and think you are trolling your team, which causes the enemy to make a lot more mistakes that you can capitalize on. The best champions to pick Janna into are tanky low mobility champions, such as 
especially, but she's also great for even more mobile champions such as 
.
You might find it hard to believe, but Janna's kit actually counters these champions perfectly. Think how awesome she is supporting her ADC in traditional support role in keeping these champs away. Now back up her kit with some decent damage and huge shields, and you have a monster to lane against for most traditional champions. With some AP, her passive W speed also makes her insanely fast in lane, so for many champs, if you W them, it will slow them to the point they can't retaliate, and if they try, your Q will stop them from reaching you. Riven, for example, is completely powerless against Janna all game unless she gets snowballed from another lane or a failed jungle gank. Janna can interrupt Riven's Q-dash mid-cast, preventing its mini stun, and with Janna's speed, the only way Riven can touch Janna is with Flash.
Janna also has a unique role in countering most assassin champions - she either shrugs off or completely ignores their initial dash attacks, or monsoons them into her turret when they try. This makes her a viable mid laner against certain champions such as 
. And when I say viable, I mean she can make them go 0/5 by 12 minutes in, all while being screamed at by their team in all chat for losing to a Janna. Many of these champions are supremely squishy early game, and Janna's shield forces them to all-in to do any damage to her, which Janna can then counter by disabling the hell out of them while they're in your minion wave.
Since Janna relies on poking over time rather than all-inning, she struggles vs champions that either sustain themselves, shield her harass, or have the ability to push extremely hard before she gets Luden's, so avoid picking her vs champs like 
and ESPECIALLY 
. If pops up on the enemy team, consider queue dodging since he removes a lot of your strategic advantage of pushing from extremely far away, and takes away a lot of your presence in teamfights. Consider Yasuo to be Janna's hardest counter.
When playing Janna top, you will usually be faced off against a melee juggernaut whom can be easily harassed with your W followed up by E-empowered auto attacks, which give the added benefit of preventing minion harass. Refrain from using your Q in harass or farm, depending if your enemy has a dash or not, it can be used to interrupt the dash and cause them to do zero damage to you. Your goal is to proc as many thunderlord's as possible while making sure they aren't able to hit you. If you utilize W's move speed advantage, you can hover around the enemy while they try to CS, then as they go in, quickly use W, and quick-cast Q if they try to retaliate. If they aren't able to hit you, activate E and auto until thunderlord's goes off. Your goal is to eventually stand between the enemy and the minion wave. If the enemy is playing extremely defensively and is staying out of your range by a lot, keep up your W buff to be prepared for a potential jungle gank.
I'm not going to pretend to be an expert at the game, but I've played Janna solo lane since pre-season 1, and she has gotten a lot more powerful in this season than she has been since her big nerf a few seasons ago to her AP Ratios and damage. I'll add in some more later if this guide gets any support. Have fun.
