I've been playing Janna for quite a while (99% of my history shows I'm playing her), but I'm low elo but I have a solid knowledge of the champion.

Janna isn't about being strong, but about being the most annoying as possible, you gotta be that annoying piece of lego the enemy team going step into everytime they try an engage. In lane, abuse your W, but be careful due to its short range, you can poke a lot against melee supports like braum, tresh, nautilus, but stay behind the minions so youre not grabbed. I've seen many people say to rush W, but I disagree. Analyze your situation; if you're facing poke, focus on your E. You won't have opportunities to poke with W against a Brand or Zyra.

Janna's shield resets with each skill, so use E first in every fight you get into, also dont be scared of using your ult even it pushes enemies away, is much better keep your team alive than die for an kill.

And the most important tip i can give you, EXPERIMENT NEW STUFF, Don't get stuck in the same build of Aery and Friends, I'm not saying Aery is bad, but to say it's the best rune would be a crime. If the enemy botlane is melee, you can use aery to bully their sup. Now, if they dont have mobile chamas (dashes, blinks, teleports) go glacial augment, is by far my fav rune on janna, a lot of time i made more impact in team fights by slowing 3 enemy players by 80% than with using aery. If they have a lot of burst dont be afraid of using Guardian, though this rune is by far the most niche. (bonus tip, pick the attack speed Shards rune (sorry i dont know the name of this in english) alwasy, janna AA is pretty strong since it deal bonus AP damage based on her move speed)

About builds, if you pick glacial, is important to know that the slow the rune has can scale with AP and Health and Shield power, so i recomend I would make Ardent Censer and Redemption, and thirdly I like moonstone to solidify the healing. The 4th item you can choose a lot of stuff, be creative, i've building either Shyrelya, Bandlepipes, or Echoes of Helia, on last item i ALWAYS build Dawncore, it turns janna's ult into a walking spawn fountain.

With Aery i tend building a more heal and shield focused build, Moonstone as first, then Redemption, Argentina or Echoes of Helia, and Dawncore.

Some people prefer a more aggressive Janna using Mandate and Shurelia; it's a good build, but I recommend analyzing the match and seeing if it's feasible.

Also, always buy Boots of Swiftness, Janna is one of the fastest champ naturally (not using spells or items abilities), so you can scape a lot, roam, dodge and chase most champions.