Hey There ! This is my first build ever and it is with Janna, the storm's fury. This is the build I tought about and that gave me a 17 games winning streak. It makes you a great support with great Ability Power/Mana/Health. Stay back, harass and help you partners and everything should be fine ;) .

First off, I will talk to you about Janna'S Pros(Why she's good.) and her Cons.(Her disadvantages).

PROS
-Nice first gank damage/bush check with the skill Howling Gale.
-Nice CC with Zephyr, who Augment your own movement speed/slows and damage enemies.
-Eye of the storm is a nice shield who can also shield allied turrets.
-Monsoon is a channeling heal and ultra push your enemies. It can save your partners lives.
-Movement speed buff of 3% with her passive, wich helps the team alot.
CONS
-Squishy early game.
-Not good to solo a lane in my opinion, you wanna have a support like Janna fighting with you.
-Not enough Mana early/mid game.
-Kinda useless against Magic Resist Tank.(Execpt if you have Magic Penetration)
-Using Zephyr to slow an enemy stops your movement speed buff, pay attention to this while low health in team fights.
-Now a goood pusher, don't deal high damages against buildings.

Now, the runes. I use these runes with every Caster/AP Support I have and it worked pretty well for me.

They give me a lot of early game AP, wich gives me the ability to deal high damage soon.

Some Magic Penetration, 'cause you never know when you can fall on a MagicResist Tank and it is still useful to deal higher damages.

Mana Regen, so I can spam more spells.(Great with Eye of the Storm so you can spam it on turrets or allies to shield 'em.

And some Cooldown Reduction, to harass them more and all the stuff.

I always used Defense Masteries with Magic-User Champion. Why? Because defense is mainly their week point. So with defense masteries, it garanty me more survivability in lane so as I stay longer in fights, I can assists my partners and make us some kills. I also use Offense Masteries that I will detail now.

I take everything that's armor/MagicResist/Cooldown as defense masteries. That way I survive longer to every kind of Champ. I also take Mender's Faith, so my summoner spell 'Heal' comes back faster.

In offense, I immediately go with Archmage's Savvy so I get more Ability Power as I level up.
I also use the summoner spell 'Exhaust' boost, making them less resistant to magic/normal attacks and slowing them .5 second more, wich helps the team to get some kills or to evade a death. Then I go with Sorcery, wich reduces the cooldown of my abilities by 3% and then I can have Archaic Knowledge, making my magical spells to 15% of the target's MagicResist wich is always useful against enemy.(Especially Tanks)

As the game starts, I immediately buy Doran's Ring, so I get more Ability Power/Health/Mana Recovery early game. I then go with Ionian Boots of Lucidity so my abilities recovers faster so I can harass enemies and help my team.(Sorcerer's Shoes is also a good choice and I recommend it against tanks) Then I begin crafting Soul Shroud with a Kindlegem(spells have even more lowers cooldown), then with Mana Manipulator(so I am less Out of Mana) and finish it with Ruby Crystal. After I like to go with a Rylai's Crystal Scepter beggining with Giant's Belt, to have more survivability.(+ Your abilities will now slow enemies even more with this item). Then I build myself a Rabadon's Deathcap, increasing a lot my Ability Power.(+155 AP, +30% of current AP). I end everything with Archangel's Staff, giving myself a bigger Mana Pool and AP. If the game is very long, replace Doran's Ring with Void Staff or another AP item.

I usually go with Exhaust/Heal wich permise me to help the team longer, to survive, to save teammates from death and to finish an enemy.

If you really are in trouble, Heal and Monsoon combined is pretty good.

Be sure to use Zephyr before Exhaust if you can, it will slow him even more and give you the chance to kill him even more.

I put a point in Howling Gale first, so I can bushcheck the bottom bush(in 3v3) and knock my enemies airborn. It is also usefull to farm minions. Let it charge so it will deal higher damage and gain more range. Then I give a point in Zephyr and Eye of the Storm so I can use all my spells at level 3. I then max Zephyr first(giving a point in my ult and Howling Gale as well). Why ? Howling Gale is a pretty cool spell, but enemies know when and how to evade it, sometimes making you waste it. But with Zephyr maxed first, it can slow more and longer helping allies and it deals A LOT MORE DAMAGE. I then finish up Howling Gale and ending with Eye of the Storm, giving 2 points in my ult as soon as I can as well.

I made this build for 3v3, I don't know if it works well in 5v5 but it worked great for me. Have fun playing Janna ;) .