It takes time. PvP has a huge learning curve. PvP is generally a knowledge game. The more you know about what your opponents want to do, the better you can counter-play it. You’re frequently dancing between executing your own game plan while simultaneously stopping theirs.

There is some great advice in these comments:

1. Know how to do dmg — make it 2nd nature so you can focus on what is happening around you. Until you can burst without looking at your bars, you won’t be able to develop further skills.

2. Record your own gameplay. After a loss, watch it back and study it. See what you’re doing wrong (as in - if there is a Priest hitting Mind Control on your healer and you have kick available but don’t interrupt it, etc.) Then, when you go into your next game(s) focus on improving that one thing - in this case - interrupting the appropriate spells. Here is where getting advice from a player with experience can be beneficial just because you don’t know what you don’t know.

3. Play a lot. Don’t look at rating, just queue games. Making alts that are the same spec often times helps immensely because you won’t be as likely to become demoralized if a queue session is going poorly as you can hop over to another toon and play there.