You have to dodge skillshots. With that movement speed you can dodge nearly everything. Poke your enemies, chase them and give your teammates movement speed using your pasiv. Renember you have additional auto attack damage on-hit from passiv, so use it espesially in early fights. Buy boots as soon as posible, it gives you ability to dodge easilly and gives you additional on-hit damage from your pasiv so its not only for movement. You can use flash ult combo to save allies or push enemies under your tower.

In my opinion Janna is good blind pick but you need to have experience and skill in this to dodge skillshots.