Hi I am Apunahu (EUW). I have played Janna more often than any combination of two other champs, maybe except for Nami+Sona. Currently I am located in lower gold on Europe West.

There are sufficient tipps how to play Janna in other guides and mostly you will learn from playing as her, So I am not going too much into splitting teams with the ult, flash-ulting, charging tornadoes where your enemies will soon have to walk, baiting, escaping and sacrificing yourself when necessary. I am not going to talk much about how you can make Leonas life a misery and I am absolutely not going to explain how ardent censer scales passives nicely with her shield. Just some small tips regarding things that I haven'T seen mentioned anywhere else:

You are fast, use it! You can catch enemies that would be safe from every other champ except maybe Lee Sin, dodge even fast abilities and easily reposition yourself. This also gives you the ability to manipulate your enemies' positioning and mabe even bait mistakes via mindgames as they try to react when you seemingly get into their range.

Pick Janna when you want to protect your adc against bruisers and buff him against tanks or when you are running a poke comp. She works against Asassins too but in my experience Lulu can peel against those better while giving bigger lane pressure early and adding in a nice bit of small burst to kill the assasin fast while her utility isn't half bad either.

Pick Janna, when you have a hypercarry, Jinx, Vayne, Tristana and Twitch all love her Shield. However Twitch probably prefers having a tank to engage as he lacks mobility to really shine with Janna and his damage is completely sufficient.

Try to avoid: Caitlyn, Zyra and Kogmaw as enemies. You will find it difficult to surpress a kogmaw during early and you will have difficulties engaging on a cait or Zyra.

One more Thing: Learn to block all sorts of engage. You have absolutely no excuse for letting an irelia, Jarvan, Khaszix, Lee Sin or Leona reach your adc unless that adc flashes right in their face.

Mobility boots, I personally don't like them, however they seem to be very appreciated among highlevel players presumably for roaming so give it a try and find out your preference but adapt your build to still reach the cdr-cap, being able to have a shield downtime of only 0.5 seconds is your goal. Also warn your adc or take him with you when you roam.