So, to start i have to tell you something, you want kills? Trust me, don't play as Janna, Janna wasn't made to get kills, Janna was made so that you win the game, to turn squishy guys into "tanks", if you have enough ap, you can get your Eye of the Storm to absorb really good damage.Trust me.

Pros:
-Great Farmer/Pusher with Q skill;
-Can really save your Team mates a lot;
-Hard to chase;
-Can Heal really good with enough AP;
-Really good chaser/ganker

Cons:
-Kinda squishy until ~mid-game~;
-If Cc'd too much dies instantly;
-If focused too hard dies; (Depending on team mates)
-Ignite really screws your ulti;
-Banshee's Veil is Like the BoogeyMan to you trust me, no knock back, immune if lucky enough to ulti....;
- Can't push Towers that well;

So, for runes as Janna i use, Mana Regen seals (ATTENTION!: if you prefer Mana Regen Seals per lvl then go ahead, i like flat ones because, i think that i need this Mana Regen at early-game, not mid or late game, so why have more Mana Regen at 18? you are going to have Archangel's, you won't be needing that.) wich are really great, Magic Penetration Marks, because, lets face it, every caster should use it, even as Janna, a supporter, you might aswell do 200 damage (Exageration!!!) to them then none don't you think? For Glyphs i use Cooldown Reduction because its pretty usefull to have Eye of The Storm and Howling Gale with really short cooldown, trust me, you can save your friends a lot with this. And for Quintessences i use a tiny bit of flat AP just for the xtra bonuses :) .

So, i go 0/8/22, i take lots of mana regen on Utility, Improved Clarity, and Improved Clairvoyance, in defense i take a little bit of Armor, some Magic Resist, and Health Regen for the more mana i have. I go defense because if you take 22 points on Utility, you won't be able to reach Archaic Knowledge, So the extra Armor and Magic Resist its not like OMG ! but its pretty useful, but if you prefer going Offense for the Magic Pen. go ahead this is a build so that you can learn how to play as Janna, not to blindly follow it.

Read this! How to farm as Janna? Easy, always use your Howling Gale ( do not spam it, watch your mana ) on minions, don't waste your mana on champions and then miss it, Charge it, fire it = money money money.

Okay then, major point of the build:
Meki Pendant and 2 health potions at the beggining, very good mana regen at beggining;
Farm minions/champions, get 1055 gold, port base, buy Tear of the Goddess, and some mov 1 boots;
Farm more, at this time if your lane partner is compettent you should have 1/2 assists ( or kills, very rare ), go base and buy Catalyst of the Protector (I buy this because when lvling this is pretty useful, always lvling always getting more mana and hp, its great!) and if possible boots, ok, BOOTS! Boots really depend on the enemy team, lots of cc like lux/sion/galio? Mercury Treads, Hardcore only Physical team? Ninja Tabi, Mixed AP/AD team? i sugest you buy Mercury Treasds. After all of that, you are probably on mid-game, and probably already have Rod of Ages, next stop? Archangel's Staff, this is, the most awesome item for casters imo, lots of mana/mana regen and AP! From then on its depending on their other team, you have a noobish ez that only buys defensive items at late-game? Get Aegis, feeling too much damaged from that yi/tryn/noc? buy Frozen Heart, that really screws them,now, last item, its really up to you, if enemy team focuses you too much, get Zhonya's Hourglass, really good, more AP, armor, and that god-like active. Have a really Cooldown dependant team? (Katarina, Amumu, Garen, etc.) Get Soulshroud. If they are noobs that can't even kill you? Get Guardian Angel, if they finnaly kill you, they need to do it again! haha. Lastly, Banshee's veil for some good Magic Resist, mana, hp, and that Extremely useful Passive :)

Look, here i explain why i go, Q/E/Q/E/Q/E/Q/E and so on, I get Howling Gale at lvl 1 because it's really got for FB, Minion Farm, what so ever. Then i get a early point at Eye of the storm, then Zephyr at lvl 4, then Max it at lvl 14. I go Q/E/Q/E/Q/E because i don't like to do, /Q/Q/Q/E/Q/Q like that because my Howling Gale does crazy damage yeah nice, but my Eye Of The Storm doesn't even absorb teemo's Poison! (EXAGERATION!) That's why.

Moonsoon is really a nice skill. It can heal very well with enough AP ( Ignite can really screw up that ). Now! Use your ulti whenever you need, to heal a wounded ally or yourself, or to escape, now, big part of ultimate, if its like 3v1 and the one is attacking you, do not die for your team to have a extra kill, use ulti, and make them go away, if they chase you again, they are dead. I've gotten lots of deaths because my team whined a lot, ignore them, trust me.

Summoner Spells are really at your choice, i warn you, Clarity is a must on Janna, you can denie Clairvoyance and go for Teleport, Flash, Ghost +/- and Cleanse if you are too afraid of the damn Cc's.

TEAM WORK! Janna isn't made for a selfish person, you need your team! And trust me, your team gets really strong with you on their side. You need someone to nuke that lets say Master yi after you knockbacked him and slowed him, you're not a dps'er you're a supporter remember, supporter means that you are on your team to support it, nothing more.

So, to summarize, Don't be selfish, you are a JANNA! a suporter! Play like one!
Items really depend on the other team, or you learn that, or you are going to be one of those stupid galio's that build LOTS of Magic Resist against a Full AD team...
Summoner Spells are at your choise, but you should get Clarity, trust me.
Farming is a really piece of cake for Janna if done right.
Hope you didn't got bored to death while reading this build.

Have fun Playing as Janna!
Demonicveins.
PS: ****** Build coming up! :)