Not Updated For Current Season
It's really more about a tank-support play style then the actual builds and what not but here are the builds i usually do. None of them are concrete solid builds since I always build just to counter the other team.
Janna Build A. basically its just the usual magic resist mesh up if its against a full magic damage team, i dont usually have to do it since majority of games will have a mix of damage types. The soul shroud is really just a last item i mean i dont usually get 6 items lol.
Janna Build B. obviously the defensive one for a team full of physical dps damage, depending on how much CC the enemy has you might want merc treads instead of the taby boots. optional items aside more sunfire capes is randuin's, leviathans, or a Banshee veil depending on the situation. I just usually stack the sunfires =/ also i dont usually get more then 2 sunfires though its like A. i just dont usually get 6 items which is why i just threw in another sunfire lol
Janna Build C. this is one of my more usual mix up's since most games will have a mix of damage. Once again depending on the situation you can replace tabi with merc boots. The warmogs are mainly for the huge boost of hp and hp regen. its usually just an item i get if i do well and just have a nice burst of gold. once again i mean i dont usually get it unless i do have a good amount of gold suddenly i dont usually have a warmogs all game or a 6th item again lol.
Lane phase- basically as you notice i always take shield first, its has a couple different uses depending on the situation.
Situation A. your laned against a melee champion without boots and little CC, because of your passive and your W, you usually will have speed advantage. Use this speed advantage to get in, throw an auto attack or two, get out. If they chase you you can easily kite them, and even better if they get behind your minion line let them get a hit on you so your minions attack him too then kite him more.
How the shield comes into play: now remember you have shield first, so nice patch of bonus hp like mordikaisers shield. when you go to hit them even if their minions attack you, you can back off scratch-less thanks to your shield. when they attack you behind your minion line, you take little to no damage and the minions will focus fire him for attacking, thus punishing them for over extending. Keep in mind you have a dmg bonus from the shield, so as you harass them you do a decent amount as such an early stage of the game, also use this to last hit minions this is important!
Situation B. your up against a range champion or a champ with CC who can get to you and punish you for over extending. instead play a bit more defensive, and just focus everything to last hitting minions for gold. wait for the enemy champion to over extend or do something 'dumb' then punish them with the shields bonus damage.
How it works: ummm pretty much that lol i mean the bonus of having the shield skill is you can get in a few more hits or be more of a threat without having to back off right away like most champions would who dont want to take damage.
Keep in mind I have clarity and heal to help me stay in lane longer to farm more gold at the start, and for survival. Also heal with shield bonus can be great for safely baiting enemies early game.
mid-late game. so as you can see i just max the shield first, then max the passive speed skill. mid game the active for W is a nice little nuke even without much AP. good for damaging but main use is the slow, great for chase downs and for running away. Best idea is to always stick near a teammate so you can help them kill with it (obviously thats what most slows are used for, i mean keep in mind this is how i use it and im just explaining basics lol.
you'll also notice that i only put 1 point into howling gale till level 15. How i see it is it uses so much mana and leveling it early is just a waste for my play-style/build because i need the mana more for casting the shield and casual slow. My main usage for it is for disorientating enemies and and the small stun pop up for catching an enemy, or use the pop up to slow down enemies when running. Keep in mind most champions will be slower then janna unless they have ghost or more then 1 speed item. (your really fast if you have FoN too anyways) so you can use your tornado to save allies who run and still get away without much threat on you. the bonus is because you can delay it you can set it up ahead and be even more safe for your self and then just time the tornado to help an ally.
Team fights. So hopefully you have an initiator/tank that can start fights for you, i usually play with people who play those. When the fight starts you want to just GET IN IT and try and pull some attention while the initiator/tank goes in, you want to slow things and disorientate with your gale, also if you face against champs that need to channel or are starting to get past the tank to hit your more vulnerable allies, you can use the tornado to pop them up and give your allies more time to take him down. Your shield is gona be mostly for your self (i know you guys are gona hate me for saying that probbaly something like "but janna is a support she should be saving others with it!" of course you save others when you need too or even better give it to a good dps who can use the extra 45 damage to devastate the enemies but this is how i play her) since i jump in usually enemies will look at the more squishy janna looking champion rather then the meaty looking tank and will focus you (not to mention your basically a support character and most people want to take out the support) and their are times because you do go defensive people get obsessed with wanting to kill you since they dont hurt you as much (but dont depend on that but it happens trust me lol.) if your build is decent (meaning your countering the enemy team) your gona be able to take A LOT of damage especially with the shield that you can keep spamming on your self. Point is to take damage focus off of others and to keep disorienting enemies with your skill.
How this helps your team. ok you just went tank if your following this build, you do **** for damage period, if they ignore you your completely useless, for the most part <.< You can take damage, you can piss off the other enemy, and you can cause them to waste CD's on you, but if they completely ignore you all you got is a wind that pops them up every 10 seconds with no damage, and slow if they chase or run. so do your best to get attention. (not gona lie, when i talk competitively thats all that i need to get their attention, a lot of the players in the community will react to it and if you can get even one person to want to focus you, your doing good.)
everything else like timing your gales, how to get attention and who to disorientate and what not comes with experience, you just keep playing and youll see how and when to do things (yea its general but i mean that IS how the game works and all)
Also being really tanky and unkillable you can push lanes pretty well by your self if your team is distracting a good number of the enemy, you can easily escape with your speed and you have great skills for escaping like your slow and pop-up gale.
Another thing about pushing down towers, your shield is a temporary BF sword, give it to either a ally who has more attack speed then you or be sure to just use it on your self when breaking towers, it helps. Bonus if you have a lot of armor and you need to tank a tower, if your in defense the shield can take 4-5 hits before breaking!
Yea i probably missed out a lot but this is really just for those who i play with and have asked what I do, hopefully it helps you though if you read this, GL HF DD (not like janna ever dies)