Hi im NinjaWithAnAK47 and this is my 2nd guide still not the best anyways ill help you get a grip on what items janna needs.

Item purchase order:
1) Sapphire Crystal
-because you need that extra mana at the beginning.
2) catalyst the protector
-because you need some extra heatlh and mana.
3) Rod of Ages
-this gives you the mana you will need and some ablilty power while increasing the amount of health Janna has.
4) ionain boots of lucidity
-its gives you some much needed cooldown reduction while giving you some extra movement speed.
5) Hextech Revolver it gives you some AP while giving you some sustain when your spamming your abilities
6) Hextech Gunbladethis will help you will AP,AD when you occasionally auto attack,and some sustain.
7) Runnan's hurricanethis is funny to use when your abilties are on cool down which can be a blessing when getting last hits on minions and champs.
8) Frozen Malletthis is good for janna not only for health but,janna is a good champion for slowing someone down and with the auto attacks the enemy champs will be increasingly slow with the frozen mallet's passive.