This build is based around Janna's secondary buff to her damage prevention shield. The shield gives attack damage which scales with the level of the shield so naturally you max out this spell first. Secondary to this spell I take the elemental bird which gives you a nice move speed buff allowing you to ditch people who can burn you faster than you burn them. I take the tornado ability last and use it defensively as an escape or pop up to give me a break on incoming damage.

Rush attack speed items.... throw on shield and attack. When they retaliate run away unless you have tornado, then pop em up and renew shield asap. I take items featuring attack speed and some ability power to extend shield duration under fire.

Crit, AD, Armor Pen... mostly crit.

I do the tank tree for the attack speed + ability power bonuses as well as the armor and MR which help u live considering you avoid HP items like the plague.

First.
Berserk Boots
Second.
Malady
Third.
Defensive choice that does dmg (Thornmail) or (Sunfire)
Fourth.
Madreds Bloodrazor
Fifth.
Sword of the divine ( I hate Jax)

P Burn people down by surprise for early cash
P Take out towers with ease
P Fast mover with fast attacks means lots of last hits

C You're gonna get made fun of at the start
C Janna is far from sturdy so you need to run from high hp toons until you have madreds
C You're playing Janna and she has no cool skins.