Lets skip the formalities, you came here to get a Janna build and this is what I give you.
I've seen different builds on Janna, most of which I've seen to be effective. Im not a master Janna just yet, but I know Jannas potential as a powerful lane partner.

Lets get started. (my runes, builds and guide is based on logical thinking, I may be wrong, but theres method to the madness)

Pros

Good harasser
Strong lane presence
Good at helping teams escape/get into a fight
Can position the enemy in dangerous spots
Can buff teammates and turrets
Can slow enemies during a chase

Cons

Easily taken down if ganged up on
Cooldowns are too high for teamfights, abilities need to be perfectly placed
Ultimate is risky if used wrong

Im not going to go into major depth, Im going to keep this very simple.

Never be afraid to steal a kill


With the rune choices here, you'll find you will be stronger in lane than a Janna should be. As a support, seen to be quite squishy and easily cut down from the herd. Ability power Gylphs & Quints make you strong ally in lane without really trying to be. Using Howling Gale to blast through minions (Now I advise dont charge up too much so you're not killing the minions if you're going to try be a full on support, if you're on your own in lane, ah go on, be cheeky, kill a few >_<)

Now the armour. Not much to say on this, I guess it explains alot. Janna is squishy. No amount of gales can save you from a 1v1 against Yi, Tryndamere or any dps for that matter. BUT, its nice to be alive that little bit longer as you wait on your team to come help you for a change.

Magic pen Marks. Well we wouldn't want that Q of yours to do nothing now would we? Its handy to have some kind of fight. You can't predict the teammate you go along side will be a high elo player, so you need to have some fight in you to make it easy for your carry to get kills.
A fully charged gale, a hint of W, and your shield on the carry, and he'll be thanking you for such an amazing help... (mana hungry yes, but its helpful to send a message)

Oh god here we go.

Now you might question my build, but like I said "Method To The Madness"

Now, i shouldn't have to tell you while playing support you have to place wards. Using my best friend google, I found an image thats sufficient in warding. If any questions on warding, Id suggest looking through the site, Ive found plenty of warding guides that are worthy of merit.

Why start with the Dorans ring? well its got AP and mana regen. Its going to be quite helpful on your teammate that you're not going to be dead weight and not any help. That shield/slow/gale needs to have lane presence otherwise, you're not Janna, you're an Urgot Fart (I dont know the toxicity of an Urgot fart, dont lecture me on it, im sure its bad to have in lane).

The Philosophers Stone

A great way to keep yourself in the lane. when going back for one of these, Id suggest getting it piece by piece just so you dont waste all the gold and have none left for well needed wards. It builds into your future build so not a wasted item.

The Locket

Its a nice item to have, health regen so you can stay in lane, a nice aura, and an extra shield for you and your friends! what more could you want? Pentakill? NO! BAD! SIT! GOOD JANNA! Be patient, you might only need to use this team fights at most, the occasional battle but never waste it. It can turn the tides of the fight very quickly.

Aegis of the Legion

Armour is a must so grab this item. This item CAN be rushed if you want. If you're with a Caitlyn and she's having trouble in lane, the armour aura will help her out alot. Now this is only an example, but this is an item that you're allowed to rush if you need the aid.

Boots

Now I have Ionian boots in the build. Most builds suggest you're having a good game. Yes this is the build if you're having an ideal game, but its never always ideal. if you feel you're lacking in lane or being hunted too much, you should get Mercury Threads. Ionian boots are there so you can dish out faster punishments, but if you're not the one dishing it out, play safe with your MT.

The Deathcap

By this stage if all is going well, wear your Deathcap with pride. This is an optional item to give everything a boost in the ancient art of *** kicking. Nothing much to explain on this, its a nice boost to have, you'll be thanked.

Chalice Of Harmony

Oh my! why is this last on the list? You got me! To be serious, its optional. I usually put it at the end of the build for one reason. By the end of the game, everyone is so kill hungry you'd be lucky to be allowed have the blue buff. With all the mana regen you have now, you'd basicly have a blue buff that lasts till the end of the game.

0/9/21

Ill explain this in little detail as it needs little.
9 points on defense because the runes give you the attack force already. Defenses should be good now once you have that.
As for utilities. Its a must with Janna.

The spell cooldowns and the money building up is needed for any support, not just Janna to stay in lane and be a good laning partner.

Now the game has begun, you'd might wanna CV the enemy spawn to see what Items where picked, see what you're up against....wait...wait....now cv. You dont want to do it right away, most people have slow connections, or maybe take time to read things, but you cant promise that 5 people will pick their items in 0.1 seconds of the game starting. Wait about 2/3 seconds. Take your ring and get to your lane.

Now see this is where it gets situational. Usually when I play, Im paired up premade. We usually have a combination of Janna and Miss Fortune. They work well. She "Makes It Rain", and Im the wind that annoys you while it rains. Good Damage and an annoying support (See what I did there?)
You need to analyse your teammate to see his/her play style. If your teammate is aggressive, you'll have to be just as aggressive (but dont steal minion kills, its ok to accidently get one or two, but 35 and you're not a support anymore). I'd say go back every 1000 gold. so you can get item parts, wards and pots if needed. Support items arent expensive if your carry is basicly paying for you with the assists.

Now I'm assuming now, you're doing good. Good work. Make sure you're up to at least 2/3 major items in. By now, laning phase is done with so now roam around looking to help. This is where your great mind comes in. Choose carefully. if theres a 2v1 against a teammate, and a juicy looking stream of minions. It can be tempting to say "F you, Im getting gold of these tiny buggers". You need to choose wisely. Q can save your teammate from getting chased. Double tap Q (this is if you have Smartcast on. Keybindings if you dont know how to do it) and it will send out a low power gale to knock up chasing enemies. Shield your ally, and run away.

The ultimate.

Its a very effective tool for helping at this stage. BUT if not used correctly, cant ruin your team. I've learnt this plenty of times through trial and error, you might think you're helping out by shoving everyone away from the tower, but if you're friend lands between 5 players, he wont be thanking you. This ulti can be used in 2 ways. A healing device and a pushing device. Your Q wont push anyone away. They go up. If its desperate you get the enemy away, never hesitate to jump into the 5v1. But be FAST on that R. They will get pushed away and your team can regroup. In that situation, dont channel your ulti. By the time they are pushed, they will already start moving at you. Push and run. If you're able to channel safely, Heal to your hearts content.

Heres where things im guessing are in you favor. If you're losing by now, never worry. With Janna being a good pusher/harrasser, even at low level can create distance between you and the enemy. Doesnt always secure you'll win, but by god it can help.
If you are winning, nows the time to consider, your build. If you needed your Chalice already dont worry, like I said its optional. If you think you need more presence get the Deathcap. If you feel you're team is doing fine now and the win is definate, id suggest get something with armour like a thornmail. THORNMAIL ON JANNA! Gasp! Nah its fine, see you dont want to be the one getting killed 24/7. By now everyone can hit hard so Janna even with what you have can still feel fragile. The last item slot is optional so its up to you. Examine the situation and go for the win.

This is my first guide on Mobafire. Feedback is essential as a learning curve for both people reading, and for me as a guide maker. Im aware it has holes in it, Not as pretty laid out (I haven't got the hang of it all just yet), some things seem flawed. This is the bare bones of what to do as a Janna. Any more information needed? Comment and Ill answer best as possible, if im unable to answer, Ill suggest a similar guide in which I feel has an answer that has what you need.

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Good Luck Have Fun.

*I do not own the images provided in this build, pictures were provided by google. If i owned League of Legends would you think Id be making builds? hell no, id be reading them and playing all day... see you on the Rift!*

Thanks to
Argonoch
azureus tigris
OhHaiThur
Naberius
being teammates that help the builds succeed

Extra thanks to Azureus Tigris and Naberius. Plenty of times we've discussed items and essential build construction and it pays off each time now.