Let's start with the summoner spells! Just like with every other support we're gonna take exhaust and flash for obvious reasons. !But! you can play her with ignite and flash, too. I am only recommending the second method if you are sure!!!! what you are doing and if you can trust your ADC (for example your duo partner :)) because Janna is a very caring protector who's not dealing much dmg to help you ADC in fights, she is a cc queen. You can compensate your lower supp dmg (compared to Nami or Sona) with a nice exhaust on the enemy ADC!
Items: I play Janna with the yellow support item, but if you prefer the blue one you can choose that, too. Sightstone for vision, shoes for more movement speed, all that ovious stuff, let's start with the interesting part!
Twin Shadows: perfect counterpart to the Talisman of Ascension! Slower enemys, faster team = perfect.
Ardent Censer: insane attack speed buff, perfect synergy with Vayne or Kalista!!
Explanations for the last item in the build describtion!
Personally I am using her Q to zone and knock up, not to deal damage with. But don't forget that your whirlwind will fly a longer distance as more as you charge it!
W= Perfect slow. / Nice poke if you play AP- orientated.
E= Most important: an amazing shield absorbing massive damage + giving an AD buff! (can also be casted on turrets!)
R= Use it to heal you team, get enemys of your team (defensive) or to push the enemys into your team (aggresive).