First of all, welcome to my guide of the Janna support, I have not written a guide in a while but felt like doing this, maybe people will find it usefull maybe they dont but this is a build that suits me quite nicely and instead of having to memorize it all I'm writing this guide.
Hopefully you will find it usefull and I'm open to suggestions to what could be changed about it .

Well for the Seal and Quintessence it is quite obvious why I chose them, as a support you're not meant to get champion kills or minion kills, so you need a steady income from other sources but if you like you can also play with seal as armor and pure AP as Quint, works too.

The Mark and Glyph are mostly cause you dont wanna be focused and killed too easily in early game so these should keep you safe, you could use others if you like I'm sure but these are the ones I've played with.

Offensive:
Summoner's Wrath: Also lowers the enemy's armor and magic resist in addition to slowing them down and reducing their damage making them a much easier target for your AD Carry.

Defensive:
Hardniess/Resistance: More armor magic resist, always usefull :D

Utility:
Good Hands: The shorter you're dead the more you can support, if the unforunate should occur and you die you wanna get back out there quickly.
Expanded Mind: The mana increase is well more mana, more mana equals more spells more spells equals better supportability (Is that a word? x.x)
Swiftness: Nice before you get your boots as it stacks with your Zephyr.
Greed: Hehe cause we're greedy :D
Awareness: More experience so you can potentionally outlevle your oponents so you can cast stronger spells to support your lane buddy.
Sage: I thought this was a good idea but ofcours there are better places to put it but extra experience is always handy.
Intelligence: Shorter cooldowns = faster shields, slows and toss up into the air.
Mastermind: More often being able to heal and refuel mana wich both is nice in panicky situations.

I chose these items based much on the fact I play personally with a ranged AD carry like Ashe, Varus or Sivir wich all need mana and attackspeed.

Also maximizing the amount of gold per 10 second you can get with the exception of one item wich helps you boost up your gold crazy fast.

Alternatives are many ofcours and it's really up alot about what you're facing I guess.

Not much more I can say on the subject.

I took these cause you wanna be able to throw your enemy up into the air during firstblood cause that one second of incapacitation can be the difference of firstblood and not.

Also focusing on the shield is majorly for if you play with a AD carry cause it boosts them with extra AD per rank and the slow slows more per rank but the Howling Gale only have that short capacitation through out it's 5 ranks.

Right so with some input from friends I have made a slight rearangement in the summoner spells and found Exhaus and Flash more usefull in fights as your companion should play with Heal and not you. Exhaust is nice to reduce the enemy's movement speed, armor and magic resist aswell as lowering their damage (Noob moment here but I didnt know it did that D=)

I have not done this yet so cannot comment, will uppdate after I've tried it. Feedback is appreciated.

Pros:
Boosts your AD carry's AD with a few extra points.
You can slow alot.
You can stop enemy's from runing.
You can knock enemies away from your injured allies with your ulti.
You can knock enemies into your turret if you use your ulti right.

Cons:
Squishy if focused.
Have to remember to buy and place out wards (Always forget to do that my self x.x)
Not being allowed to kill anything but just watch.
Can accidentally knock enemies away to safety if you use your ulti wrong.

Very good team work champ if you play right, can stop entire group of enemies from runing can slow a injured enemy can also heal the entire team or shield the most injured or boost the AD carry(s).

Farming? Son you're a support you dont do farming, you just sit back and watch the carry do it for you :D

That's all from me, I hope you've enjoyed this guide, I hope you find it useful, if not, well maybe support isnt for you or your play style differs from mine, either way I wish you good luck summoner and blow them away with your mad supporting skills!