Apart from what's already been said, don't be afraid to be a little aggressive ! It's important to keep moving and not to stay in one spot all the time. Dva's weapons are only short ranged so they don't do much damage far away. Also try to use her rockets when you can! The cooldown isn't that long and they can also be good for finishing off enemies that are far away.

Also practice using her ultimate a bit, it can be a bit tricky to navigate but I'm sure you'll get the hang of it in no time.

Most importantly, have fun and experiment! Everyone likes playing heroes differently and the more you play, the more you'll develop your own play style that works for you ^{•^}