As i Janna player i decided to share my knowledge and expirience that i am going to share.

Janna is a support champion with incredible movement speed with also effects her teammates with her passive. Also she has a passive for her alone when you start lvling her W which she also can use it against other enemies.
With her movement speed she can roam pretty fast trough the map and ward important areas like dragon, baron and other buff monsters. but the most important is the locations in the river to avoid ganks.
Also her shield is a very strong ability shield yourself shield your ally shield your turret it can be a life saver but it also buffs the target with more attack damage. So have a friend that you can assist more effecïently by letting him chose a AD champ for the extra damage output when you cast the shield.
Just like every support you don't need that much of minions or kills assists matters
Ward Guide

I have 2 builds for Janna a half agressive half support build and a full support build so don't get them mixed up.

As the runes for the agressive/Support Janna.
I chose flat magic Pen marks just to break their defences early game with full tornado's In combination with flat AP quintessences. The Tornado's have a far range through a lane so harras.
Also with the cooldown glyphs you can spam with tornado's making it easier for your AD lane buddy to kill if he is patïent enough to find the right place and the right time to strike when you hit with your tornado.
Also the AP runes give Janna's Eye of the storm more shieldpower and u can use it faster after eachother so you will have the shield most of the time to rescue your valuable teammate.
And flat Mana regen runes because when spamming tornados you need the mana regen

With the 9 points in offence and 21 in utility it will work with your runes together.
you will have more magin Pen and AP just like i said in the recent chapter spam the tornado's and let your lanebuddy observe and strike at the right timing.
plus you have health and mana regen and the reduced Cooldown for summonerspells.

As basic item I have Meki Pendant you can use Pots or a Pot with a ward depends enemyjungler or no enemyjungler.

Hold as long as possible in a lane to get enough gold for tear of the goddess because you will stack mana also buy some wards ik you think 2 wards enough buy directly boots of speed.

next time B or dead Boots of lucidity the cooldown reduction and mana stacks allows u to spam more tornados.

The next step is all yours aegis of the legion or start building spellvamp. Why spellvamp? Tornado in the minion wave also with an elixir of brilliance you can wipe a whole minion wave in 1 blow! (do this only when you have too Supports don't need alot of minions or kills)

but make sure you build the aegis of the legion for more survivability in-game as i didn't set defensive runes

if you have money left or you know the enemy team has wards buy oracles elixir buy it after Aegis of the legion because you will die quick if they see that you are destroy their wards and it will be waste of money if you die.

My runes for the complete support janna.
Just like the Agressive/Support Janna the flat magic Pen marksjust for early game harras tornado.
My quintessences are movement speed you will be flash. Superspeed!
The seals are the same with flat mana regen.
and my glyphs flat Cooldown reduction now it is spamming with shields.

As a Full and speedy Support i've put 7 in defence and 23 in utility the reason why is the Cooldown, HP/MPregen and defence you can also take 2 points of Vigor to put it in swiftness to have more bonus speed.

Just like the Items Agressive/Support you start with meki pendant with pots or ward and a pot.

when enough gold build tear of goddess and some wards with even more money from assists buy boots of swiftness.

when time elapse buy boots of swiftness and more wards and maybe an elixir of brilliance to push 1 or 2 minion waves at one with a tornado of you are getting pushed too hard.

Then buy Aegis of the legion as fast as you can to support your lanebuddy with the aura and the aura affects u too. don't forget to buy some wards too

Then buy shurelya's Reverie it has lots of HAP/MPregenand can be activated to give allies close to you some speed too. or skip this step and get the next item the reason why is because most people forget to use it a crucial moments in battle to run away if you can use it well pick it first if not skip it

Then buy the force of nature to get some magic resistance and extra movement speed fast warding!!! and also HP regen!!(my thing is the speed)

make sure you build the aegis of the legion for more survivability in-game as i didn't set defensive runes

with the money that you have you can buy oracles elixir to destroy wards of enemies. buy it after Aegis of the legion because you will die quick if they see that you are destroy their wards and it will be waste of money if you die.

What i mostly pick all the time is Clarity and Clairvoyance. But i've recently read a trick with flash so you can use it as well(look at Advanced). but the most important is Clarity because if you spam tornado's your mana will drop zero and just those times you need mana is the time you need to save your teammate if he's getting critical damage. but if you have a jungler Clairvoyance is nice use it to watch baron/dragon/buffs make sure u know where to use it and when. For example if you have a jungler and the enemy team has a jungler too clairvoyance can help your jungler to run through the jungle safely at important locations. also use it at river bushes if you suspect a gank from the enemy player it can be the bush at your lane or others so you can assist even at the other side of the map.

Let's begin to tell about advanced tactics with Janna.
Just like i menioned wich clairvoyance that you can support even at the other side of the map you can counter jungle. Clairvoyance at their blue buff when they are busy killing it steal it just like with baron or dragon they take the hits for almost killing the monsters and your team goes away with the bonus.

Clarity can also be supportive for example a morgana mid and you are bot with a good AD carry Bot kills both enemies and mid needs mana quickly ward the river bush and go further to give morgana some mana but do it very stealthy like wait in the bush for your morgana comes to you together in the bush use clarity and go back the enemy thinks morgana used clarity her but it is yours!

Like i mentioned in the previous chapter flash can be used by Janna you can escape over walls but there is a trick with a combination with your ulimate monsoon. the trick is to flash to the right spot and then use your ultimate the reason why this combo is you can place janna good you can split their team or you can go at a distance where you don't push enemies away but only heal your allies. for example a enemy wants to escape flash a step further your enemy use monsoon to push them back to your tower. another one flash into your enemy and monsoon enemy champs are now seperate and you disrupted their lineup so your team can go pick 1 enemy first before the team gets to eachother to help eachother.

Also you Q Tornado when you are running away from chasers direct the tornado in the dirction where you are running you can set your tornado longer for some more power and use Q again to release it when he passes by.

Your E gives attack damage to the champ you selected it is a lifesaver but also the major buff that kills os use it wisely in teamfights when the focus your AD carry.

have fun with janna and experiment with items and the way u use your abilities any tips and/or suggetions are appreciated.