Hello and welcome to my Janna guide! This is my third guide and I have alot to improve so please leave constructive criticism in the comments below! Thank you for looking at my guide and enjoy. :)

Pros
Great CC!
2 AOE CC!
Big shield with an attack damage boost!
Nice movement speed.
Long range.
Cons
No heal.
Very squishy.

Good

Clairvoyance is the main support spell, it lets you see an area on the map every 60 seconds. This can be used to scout bushes, find the enemy jungler, see if Blue or Red is not taken, foresee ganks and check Baron or Dragon.

Heal was improved so now it is taken by many supports, it is especially good for supports like Janna or Lulu because they have no built-in heal.

Flash is your main escape spell because you do not have any escape ability. Flash can also be used to initiate with your Flash + Ulti combo.

Exhaust can be used in many ways, especially against an enemy AD carry because it reduces the attack damage dealt by 70%. This can be used when you and your AD carry is facing off against theirs.

Okay

Clarity is useful for sustaining in lane mana-wise but is not needed unless you plan on harassing a lot or simply wasting your mana on pointless shields.

Ghost is good for escaping and initiating because it allows you to pass through units and gives you a decent movement speed buff. However you do not need this unless you are against a team full of slows and plays extremely aggressive.

Ignite is good for either finishing an enemy who is about to escape or reducing healing effects. Ignite is only worth it if they have a lot of healers like Soraka or Dr. Mundo

Bad

Teleport shouldn't be used by the support because it is only used to gank or get back to your lane quicker to not lose experience. The support does not have effective ganks and does not need as much experience rendering teleport useless.

Revive brings you back to life instantly when you die. A support like Janna stays behind the tanks to keep supporting the team with shields, auras, heals and CC so you really should not be dying if you are playing a support correctly.

Surge gives you a good boost in ability power and attack speed. You do not need any of this because you barely do damage to the enemy team so there is no point of any extra damage.

Promote makes a minion stronger. All this does is make it harder for your AD carry to last hit minions.

Cleanse removes all CC on you. You will not be CCed because in teamfights you will be staying at the back of your team and not in the front where all the action is happening.

Smite is useful for jungling because it does true damage to a minion or neutral monster, this is very useful to make sure the enemy doesn't steal buffs, Dragon or Baron. This spell is usually carried by your jungler and you do not need it.

The Offense Tree is for the cooldown reduction and little bit of ability power for a slightly bigger shield

The Utility Tree is for mana, more gold, extra movement speed, cooldown reduction and more experience because you are going to miss alot as you will be warding, helpng other lanes etc.

Janna is extremely squishy so she needs alot of defense in runes. That is why the marks and seals are health and glyphs are magic resist per level so when the enemy carrys becomes strong you wont die instantly when they jump on you. I also get gold/10 quints for more gold.

Increases the movement speed of all allied champions by 3%. This bonus is lost while Janna is dead.

Honestly, this is the best passive in the game. This passive is equal to 2 Quintessence of Movement Speed making it a very fearful passive.

Janna summons a mighty whirlwind. She can activate the spell again to release the storm. On release the storm will fly in the direction it was cast, dealing magic damage to enemies and knocking them into the air. The damage done, knock up duration and distance traveled by the whirlwind increase for each second it channels to a maximum of 3.
Range: 1100-1700
Knockup Duration: 0.8-1.1 seconds
Cost: 90 / 105 / 120 / 135 / 150 mana
Cooldown: 14 / 13 / 12 / 11 / 10 seconds
Magic Damage: 60 / 85 / 110 / 135 / 160 (+0.75 per ability power)
Additional Damage: 25 / 30 / 40 / 50 / 60 per second charging
Maximum damage: 135 / 175 / 230 / 285 / 340 (+0.75 per ability power)

Your aoe knock-up ability, it has massive range making ganking and fights easier for the team.

Janna summons an air elemental that increases her movement speed and enables her to pass through units.
Movement Speed Bonus: 8 / 10 / 12 / 14 / 16%
Janna launches her elemental to deal magic damage and slow an enemy's movement speed for 4 seconds. The passive is not active while the ability is on cooldown. ■Range: 600
Cost: 40 / 50 / 60 / 70 / 80 mana
Cooldown: 12 / 11 / 10 / 9 / 8 seconds
Magic Damage: 60 / 115 / 170 / 225 / 280 (+0.8 per ability power)
Slow: 24 / 30 / 36 / 42 / 48%

This ability makes you really fast and lets you pass through units so you don't have to worry about getting blocked by your minions. It also has a slow which can allow your team to catch up to the enemy you are chasing.

Janna conjures a defensive gale that shields her target from incoming damage for up to 5 seconds. While the shield holds, the target will also gain an attack damage bonus. This ability can target turrets.
Cooldown: 10 seconds
Range: 800
Cost: 70 / 80 / 90 / 100 / 110 mana
Shield Strength: 80 / 120 / 160 / 200 / 240 (+0.9 per ability power)
Attack Damage Bonus: 14 / 23 / 32 / 41 / 50

A huge shield and bonus attack damage for the AD carry. In my opinion, this is the perfect support ability.

Janna knocks surrounding enemies back and channels healing winds, restoring health to nearby allies each half second, for 4 seconds.
Cooldown: 120 seconds
Radius: 725
Max. knockback distance: 875
Cost: 150 / 225 / 300 mana
Heal Per Half Second: 35 / 55 / 75 (+0.175 per ability power)
Max Amount of Healing: 280 / 440 / 600 (+1.4 per ability power)

An aoe knock-back which can change the pace of a team fight immediately towards your favor. However, used badly and it could turn the tides towards the enemy's favor. This can be used with your flash to catch enemies out of position. It can also be used to heal if you have no other use for it.

Early Game

sight ward sight ward sight ward sight ward

At the start of the game, you want to put your wards in your own jungle to protect Red or Blue from getting stolen or you could put it in the enemy's jungle to see where the jungler is. During the laning phase you should put a ward on the brush coming from the river to prevent ganks, they can also be placed in the lane brushes to give sight of enemies in the brush.

Mid Game

philosopher's stone sight ward

Mid game is when you should have an oracles. This is to destroy the wards placed down by the enemy team making them less aware of everything going around them. Philos and HoG give you a decent amount of gold and boots of mobility lets you move around the mapper quicker. Of course you still have to ward.

Late Game

Late game is when you should be quite tanky and have alot of auras like Aegis of the Legion and Soul Shroud. You should be clearing out wards all around the map and warding all around. All you have to do is be a good support and buy as many wards as you can.

Items you can get

A cheap item which gives good stats and a significant movement speed boost either to escape or chase.

A very anti-AD item, it is very useful in teamfights for shutting down the enemy AD carry for a few seconds which can be vital to winning a team fight.

A very cheap item with another shield and some good defense and aura.

zeke's herald If you have two or more ADs in your team it is very worthwhile getting this so they have more lifesteal.

Like zeke's herald, if you have two or more APs in your team you should get it.

If the enemy is a heavy AD team then this item will help greatly during teamfights.

Soul shroud gives your team cooldown reduction and mana regen, this is for a team who has long cooldowns and are very mana dependant.

If the enemy team has alot of AP you can stay alive with this.

If the enemy team has alot of CC then you should get this to counter it.

Is very effective against a mostly AD team with heavy hitters sush as Tryndamere.

White - Red/Blue/Jungle wards
Blue - Dragon/Baron/Gank wards
Red - Lane wards

Thank you for reading my guide from start to finish! I really appreciate it. I hope you like this guide, if you dont, please leave a reason why so i can improve it. Thank you!