My advice starts with: give yourself time to get used to the mechanics of PvP battling.
You are completely anonymous. Just hop into the queue, pretend that the image you are seeing is actually the Unknown Tamer that the game has generated to trick you, and focus on making accurate choices.
You get instanced and transported, which is disorienting,
Then you have to quickly size up how your team matches up against the opposition team. Your opponent is doing the same. So which of your pets do you want to meet which of his? and what pet is he going to lead with? so what pet are you going to lead with? This decision is critical: it sets up the rest of the battle. Coming from PvE, it's hard to remember that your first pet isn't necessarily the one you will lead with, and your first move is to choose a pet to lead off with.
So your opponent has S/S Anklerender, Valk, P/P Crab
and you have Idol, Teroclaw, P/P Starlette.
What's his plan? How's the battle going to go? What does he want to lead with? After seeing your team, will he change his mind? Where do you want to take the CoD & Haunt? How do you want to take down the Crab's health + heals? What do you see as the ideal matchup for each of your pets?
Take your time, what you have of it, to choose your starter. If you know your team, and you know what the opposition team can do, you will make a good battle of it.
Sometime around the second pet you need to be asking yourself what you want the last matchup to be. Maybe you will backline your pet because you want to save it for the end. Most PvP battles come down to the third pet, so a strong finish is commonly the difference between winning and losing.
You will lose at the start. Quite a few, actually. Just getting used to the mechanics will take you several battles. That's fine; pay your dues learning the ropes.
Here are some videos showing some battles with strong teams to give you some ideas.
Discodoggy: The Very Best, parts 1 and 2
https://www.youtube.com/watch?v=MPMelI9OZ5Y
https://www.youtube.com/watch?v=fDzfQbIDk6o
Llennoca: All Tier 1 Pets in Legion
https://www.youtube.com/watch?v=DUzIwmvr27w
The Best PvP Teams for Legion
https://www.youtube.com/watch?v=aDpvLn96ea4
Rosqo: My Best Tier 1 Teams
https://www.youtube.com/watch?v=lc8o_a0deBA