Not Updated For Current Season
Janna is a very powerful champion if built correctly and played correctly. Your main spell will be howling gale, this is very important the difference between a good Janna and a bad Janna is positioning. The gale will fly through walls so if you see someone start up a gale then run around the wall and hit them with deathfire grasp taking 30% of their hps or 200 damage which ever is more. Throw zephyr, then the gale should land, then follow up with ignite. With your shield on you, your damage is also increased which should make for a easy kill. Your primary job is holding a lane, while you are there very few teams can push early as you can crush a wave of minions with 1 howling gale. Just remember to keep your distance and let other classes go in first you aren't a tank. When you see runners it's your job to catch them with either Zephyr/ignite/deathfire grap or a quick "q q" which will send a fast gale hitting them and stunning/slowing them for a moment. Shield your teammates and use your AOE heal and clarity for maximum mid game pushing, you will be able to revitalize a push when everyone is out of mana and hps and the other team is coming back with full hp/mana people.
Item selection is very very important with janna, we are abusing her strengths, much like a tank would abuse getting massive hps.Your speed runes and zephyr will be fine/comparable to others so you won't need shoes right off the bat. You need deathfire grasp early game for the mana regen/cooldown (she has long cooldowns) and the burst damage for getting assists/kills. Janna is one of the fastest chars out there and by getting movement speed runes you are making yourself into a speed demon able to catch runners and slow them so your team can kill them, with your shield and gale you should be fine. If I midlane (which i do alot) I max howling gale out first, if i'm laning with someone else then i try to get eye of the storm up to level 3 fast as possible then max out howling gale. Keep yourself shielded at all times when you are alone! This will save you alot.