This is my Janna build. I'm terrible at discussing tactics and the like so I;m just going to bullet point everything. The last 2 items in my build change from game to game a lot but the ones shown are the most common ones I use.

Pros:
-Insane farming ability
-Your team mates will love you
-Decent health and speed will usually allow you to escape
-Your howling gale can be positioned to force the enemy back and harass
-Long range on howling gale can keep you safe early game

Cons:
-Fragile, especially early game, and your a target.
-Your a support, don't expect to get many kills.

Tips:
-Go into team fights last, but don't get isolated or left behind
-Use your shield on your carry as he goes in, the duration is long enough.
-Wait on your ult until silences have been used.

-Magic pen should be obvious
-Mana regen, cos you need it
-The full 40% cooldown reduction & some more mana regen
-Ability power, because it gives you a much needed boost early game and even makes a difference late game.

Start with meki pendent, then get archangels followed by the boots. then get a kindle gem followed by the rod of ages. Finally upgrade your kindle to soul shroud. If you get time then the Sceptre and Rabaddons are bought after this but you usually won't get both built.

Against caster heavy teams:
Grap a veil and a FoN as your last 2 items

Against carry heavy teams:
Grab a Zhonyas and a Thornmail as your last 2 items.