Be aware of your surroundings
Where is the enemy? Where are you friends? What enemies and friends do you have? Who do they target? Who's healing? Who's in a bad shape and needs support (friend) or ripe for the kill (enemy)? There are many ways to setup your UI and your camera to help your here - which one works best for you you'll have to find out for yourself.
Don't hesitate to ask for advice.
If you see someone really good, ask him for advice. Be friendly. You will find many that will reply in kind. If they don't, that's their personality flaw, not yours!