The biggest question I usually get asked is why I choose to Max E first.
Simple. Janna is an Enchanter. She will always be her strongest when playing to her strength, which is shielding. E-Max was always the norm up until about Season 8 when W max took the center stage, but ask why go W-max when you can just play Zyra, Brand, Morgana or Sona? Janna's strengths aren't in her damage, and while her poke can be good, it falls off very hard.

Guardian
Guardian and the Resolve tree in general gives you so much more utility than the Sorcery tree in laning phase.
Aery
Aery making your Shield bigger in early game and and sorcery tree giving you more mana a little bit bigger damage in lategame and making your heal from ultimate so much bigger
Comet
Imo comet is only good option in early game cause comet making your poke bigger but that's only this.

In Teamfights, your priority is to peel for your carry. Whoever is doing the best, that is your carry, don't babysit a 0/5 ADC who doesn't know what they're doing, beef up that 4/1 Yi and protect him at all costs.

As an Enchanter, you don't provide much damage, you need to make an impact elsewhere. The good thing about Janna is her disengage potential and amazing map presence, you are very fast and can disengage well if you happen to be caught out while sneaking in and out of the enemy's jungle placing deep wards. Vision is your biggest asset.

I hope you enjoyed this guide and I hope it helps you to be better janna.