She seems like a waste to go AP at all unlike Sona or Soraka, and I've never really built a proper "support" before :/
What do guys?
Mana Man-> Kage's
You're not even strong enough to kill my boredom
Probably not the advice you're looking for but your build is really dependent on your team.
Locket, Shurelya's, and Aegis are all great items and I recommend getting them in that order if you can. Last slot is completely situational. Rush a Philo stone and a Sightstone and build accordingly from there.
Your typical support build atm is Faerie Charm into Philo (wards), sightstone when you can (but keep buying regular wards - I cannot emphasize this enough). Boots, and then it just depends.
Runic Bulwark and/or Locket are good against AoE comps, for example. Shard of True Ice is nice for hard engage comps (stuff like Malphite, for example).
Now playing: League Of Legends (FireAwayX), Guild Wars 2 (Floralei/Crownbird), World of Warcraft (shut up), SFxT (Poison/Nina, Juri/Lili)
This is my Janna build:
Start with Faerie Charm, 1 pot(hp or mana, whichever you prefer) and 4 wards
Rush Philosopher Stone and Kage's Lucky Pick
Boots of Mobility or Ionian Boots of Lucidity (I prefer speed, but some people prefer CDR)
Aegis of Legend/Runic Bulwark(if no one else gets it, if someone else gets it, don't get it)
Games rarely go pass this point, but if they do, at this point in the game I prefer to just build AP to help the team win
Rylai's Crystal Scepter
I wouldn't get Rylai's Crystal Scepter, like the last person suggested she has a slow and a knockup already. As these are who only 2 attacks that do dmg you only gain the benefit from the knockup and as there in the air for some of the slow time its really not a good choice.
I would get the Rylai's because it gives HP and AP, and the slows stack, so it would slow them even more with W, but on the last 3 items, you'd get whichever you prefer
You can also try to get a Tear of the Goddess earlier in the game and start to stack it to change it into a Seraph's lategame if you get to it, since Janna has the biggest base mana pool in the game, 1454, along with the 1000 you get from Seraph's, you'd get 133 AP from it, along with some more Mana Regen and the shield
Will of the Ancients is another good AP/Support item since you'll give Spell Vamp to everyone and more AP for your AP Mid