Janna is a wonderfull support that is very versatile and can dominate allmost all matchups on botlane but you may ask your self: "why janna toplane?" well there are multiple reasons:

1. Your a hipster. Janna is almost never played toplane and playing her looks cool.

2. Suprise. because nobody plays Janna top nobody is used to play against her as well wich gives you the advantage

3. Fun. Playing Janna toplane is Fun. It may require a little bit of practice but you can dominate allmost all matchups because toplaners usualy rely on their jumps when playing against ranged Champes.

4. Rage. Everybody knows lol is a toxic game. So imagine you are playing toplane against a ranged Supporter that totaly destroys you. Most players would get mad and start playing worse in addition to blaming riot games for bad balancing. In addition the other enemys will also get mad and stard throwing, because their toplaner looses against a Support.

1. Hypercarry

If you play janna toplane you are missing a tank in your team wich can be a problem.
However you have two supports, so a hypercarry comp is suggested.

2. Canceling Jumps.

It is importent to Cancel your enemys jumps because janna has only little health and can be easily killed if you reach her.

Flash + Ignite are pretty standard providing damage and mobility

Flash + Ghost:

You have to use this against champs that can catch up to you easily (e.g. Nocturne) or if you want to have an easyer time chasing or escaping from ganks.

Ghost + Ignite

Take this only if the enemy has a hard time getting to you as you cant use flash as an escape
However the damage increase you get from ghost can give you nice kill presue espacialy on first levels

Starting Items

Corrupting gives nice sustain and damage, even more when using time warp tonic
Tear for more poke so you can spam abilitys. Later it will provide lots of AP
Boots let you deal lots of damage and also help with dodging

Mythic Items

I would allways recomend building Mana Mythic items as I allways build tear.
Ludens is good because of the splash damage it deals and the movementspeed it provides.
Liandrys is good because it provides lots of abilityhaste wich will let you spam your ablilities Lategame.
It also syergises well with Cosmic drive, wich will get changed soon anyways.

Boots

Boots are an extremly important part of jannas build because they also provide damage in addition to mobility.
Boots of swiftnes are good because they provide a good amount of speed and damage permanently and prevent you from geting sliced by nasus and other extreme slows.
Ionians let you Spam abilitys wich is really cool. they also synergise with Cosmic drive even if only a little.
Moblility boots provide lots of damage on your first W or AA wich can be extremly cool when playing support.
However it ist harder to make use of it when playing toplane because you will also have to farm
Mecurys and Plated boots are selfexplanatory.

Legendarys

All of these items have a good use.
Liandrys and cosmic drive provied utility and defense
Rabadons in combination with Seraphs provide lots of AP and damage
Horizo focus provides lots of damage because you are quite CC heavy.

I also one to two supportitems because you want to support your Hypercarry as good as possible
Ardent and Staff provide a little healing as well as AP while Mikaels and redemption provide lots of heal.
Keep that in mind but focus more on the effects

Max your W first as it will give you lots of damage and E second.
A level 2 Q skill is usualy good because you can cancel enemy lvl 2 engages leaving them exposed to your damage. However you can also skill your E if the enemy has a longrange pokespell as well.
Max your E second for supporting ganks, impact in teamfights and some more sustain in lane.
You can also max Q second for some extra damage.

Arcane Comet

Arcane comet is extremely strong because of the extra damage it can provide. When poking the enemy Arcane comet can give you the edge and generate easy kills on first levels.
Relentless hunter is quite good for poking as well because of the extra Damage you get on your W or autoatack also you can roam midlane fairly easy due to the extra movementspeed.

Theese runes are strong against champions that can close up to you fairly easy such as Renekton, Mordekaiser or Darius as you can deal lots of damage in short trades, wich leaves the oponent with little time to react.

Summon Aery

Take Sumon aery against Champions that cant do a thing against your poke such as garen, maybe a lee sin toplane or illaoi as you can cancel their engage with your Q. The damage over time your aery provides is quite nice and its shield is quite strong in teamfights. Biscuit delivery is super strong because it provides sustain due to its mana regeneration, wich you will need because you are permanently spaming abilitys.

For both runepages you may also take waterwalking for roaming midlane and fighting for crabs as you will likely have priority.