Hello, I am Jahoosawan and this is my first guide on this website (hooray!). I have been playing this game since mid season 1 and I've been playing Janna regularly since season 3. I got myself to Platinum playing Janna (and Nunu). Also, it's definitely worth the money to get Forecast Janna. I am writing this guide as a way to distract myself from finals, so it will probably end abruptly once I realize I'm running out of time.

Pros:
Super safe
Powerful shield
Ridiculous mobility
Best taunts on Forecast Janna
Easy support to learn

Cons
Really squishy
Tough early game
Has a hard time engaging
Can't apply a lot of lane pressure without ADC being in synch

I put a lot of notes in with the item guide, masteries, and rune guide, so I won't spend a lot of time on that. Runes and masteries are things you should develop for yourself once you get the hang of a champion, and you should also change them based on the matchup. I like to run a movement speed Janna that has powerful shields, hence the item purchases. To be honest, I've only purchased Redemption twice, and used the active once (killed support Nunu with it, score!). I love Ardent Censor (easily my favorite item on Janna) because it boosts shield power, gives CDR, and provides an attack speed bonus when healing or shielding. Janna already provides an AD bonus on her shield, so she will also give an attack speed boost with Ardent Censor. Not to mention the entire team getting an Attack Speed boost during a well placed ult in a team fight.

This is where I have the most difficulties with Janna. If you're fortunate enough to be able to hide your pick, you might get a nice lane. However, poke supports and sustain supports will beat you in lane (based on evenly skilled players). Janna really only does well against hard engage supports in lane (Alistar, Leona, Thresh). With that in mind, you need to follow your ADC's playstyle. If you have an adc that likes to trade, make sure you put a shield on them to give extra AD as well as a shield to absorb the trade back. If your ADC wants to sit back and farm, use your tornadoes to keep the enemy off and use your mobility to distract them. Don't be afraid to use auto attacks either, especially before level 3. The shield's ad bonus makes your auto hurt even more than most supports, while also shielding their counter attack. her W isn't quite as useful in lane, other than giving yourself more mobility and a desperate slow on a ganking jungler or roaming mid. Her ult can be a real game changer though. A well placed ult can separate an ADC with their support, or isolate a jungler that's ganking. A flash ult can accomplish with high success rates. You can also push an aggressive adc under your tower for a couple tower hits with a good flash ult. Basically, what I"m saying is to not ignore the fact that your ult pushes people away from you. The heal is powerful, but the knockback can be even more powerful in the right situation.

Janna should always (Only a Sith deals with absolutes) be with another champion, most preferably a carry. She has the ability to keep one champion alive through a heavy barrage of engage. Shielding is the most obvious way of keeping a champion alive, but her ult and tornado also act to keep enemies off your carry. Essentially, you are your carry's guardian angel and you can keep them alive for a good few seconds in a 2v2 or 2v3. You shouldn't be in the front line unless you have absolute faith that you can survive whatever cc they throw at you (A.K.A. all abilities are off cool down and your summoner spells are up). <tangent> Sometimes you'll face a team that just likes to kill you as the support, and I usually oblige in those situations. I put myself in a slightly more vulnerable position so as to draw their skill shots, which will most likely miss because Janna has ~500 movement speed all the time. If they do land the skill shot, then you use all your abilities to keep yourself alive while the rest of the team punishes them for engaging on Janna. You may die, but the rest of your team lives. </tangent> However, your main motivation in life as a Janna support is to keep your carry alive. You do everything you can to keep them alive so they can remain in their bad position and deal tons of damage.

Don't farm, this is why I suggest gold/10 runes. By the 10 minute mark, you can purchase two control wards from the passive gold, with 30 gold to spare. Or in other words, you got enough free gold on your first back to get a control ward that you otherwise wouldn't have the gold for.

It's reaching that point where I need to do productive work, but I hope this guide has been helpful. Just remember to hover your thumb over the Alt key all the time so you can quickly self cast your shield on top of you. It's saved my life at least a hundred times.