In lane, you shouldn't waste your abilities randomly. Use E when people trade, ideally while projectiles are in the air. This may not be possible depending on ping and reaction time but if you can, wait for the projectile to be fired and then shield. E is your main ability for trades. The rest should be saved unless it's an extended trade or all in, but the ideal trade with Janna is just a few autos that you win because of shield. E is much better on adcs than on Janna, a Lucian's rotation has a much higher ad ratio than a Janna's. That being said you can shield whoever is trading, and if they go for you instead of your adc you can go ahead and shield yourself, so it's situational. Poke with autos yourself when you can, such as during a trade or when they can't trade back.

Also when your jungler is gonna gank you can play more aggressive; like when your jungler is close you can w and q them to make them immobile or if you want to be a little fancy w,q them then flash ult them (Highly advised to do when they don't have flash!!). As Janna you're gonna wanna try to win the bush on either side but mainly its best to stay with your adc outside of the bush. If the enemy jungler is ganking and your adc has no sums time for you to take one for the team! Make sure to save ults for when then enemy jungler is ganking or if the enemy engages for example a Leona and thresh have good engage but if you dont q on time there's a high chance they will all in you and your adc, so just stop them by ulting.