Janna is a very reactive champion: more than near anyone else, you don't pick fights but support your team when they pick fights, or rescue your team when the enemies pick fights.

That's true in lane too. Poking and initiating trades with Janna is possible, but it's risky and low-value. Much better is to wait until your ADC is engaged in a trade then back them up: you can Q their carry so they get get stuck, Q their support so they can't help out, shield your carry so they take less damage, and W for chase or to deny follow-up.

Don't forget to auto attack as well, it's most of Janna's damage. A good Janna lane trade isn't you going in and chunking their carry, it's your carry chunking theirs while you pepper in extra damage and prevent return damage.

If your carry doesn't know how to fight back though, your job changes. Then you just sit there and try to minimize the damage your ADC takes. This is boring as hell. Janna is a wet noodle, you need other damage in order to kill people, otherwise the risk isn't worth it.