When you are playing ranked with Janna make sure you identify what type of support and ADC you are going against. This can help you determine if you want to play Janna early on with an aggressive Spellthief's Edge (
) or go more conservatively with an Ancient Coin (
). Typically you can go aggressive against any tank champions like
,
,
, or
. These champions lack the poke to counter your poking, but be aware that these champions will be able to smother you sometime shortly after they reach level six. Luckily if you are level six as well your ultimate serves as a great disengage tool against any attempts to dive you or your ADC.
Janna outside of lane phase is virtually useless without a companion so don't wander away from your teammates because you are far to squishy to be alone (and if you are building any sort of attack damage item that may allow you to duel with other squishy champions, you are not fulfilling your role as support).
I attached two diamond level support guides for Janna from www.lolking.net.
Hope this was of some use.
http://www.lolking.net/guides/246584
http://www.lolking.net/guides/78745