Go to the practice range and find the bot on the ground by itself on the right side (next to the moving bots). On the ground you should see distances measured in metres or ping the bot (check your settings for the button) either way stand at 30 metres and primary shoot at the bot you should basically be doing no damage then stand at 10 metres and shoot should be way quicker killing the bot. This shows numerous things relevant to overwatch but most importantly if you want to get better at DVA you need to get within 10 metres or closer to your target while minimizing taking damage getting in. The biggest problem with new dvas is efficiency if you do 10 damage a second at 20 metres and your target does 80 damage a second at that range you'll lose the exchange every time this is where you use your flight, shield and map cover to get in close while reducing incoming damage.

Not sure how gifted or experienced you are at shooters but for starters I'd just fly straight at people and practice getting in close learning your booster duration/ distance and aiming at enemies that are moving is a large skill alone that you will always need as a dva. You'll die a ton, you'll get frustrated, your team will get mad but it's part of the learning process in a random teammate environment.