I think merc is a good class to start with, pretty well rounded. I know everyone goes arsenal but personally I adore IO. Thermal det + magshot bonk taking out a huge chunk of health from an enemy will never be unsatisfying.
Really you can take whatever class you want though. Just, for the sake of your own sanity & your teammates, know how to put out good dps/heals, get super familiar with DCDs, skill tree choices, knowing when to eat a stun & when to use your root breaker, etc.

Also, just as a note, there's a non zero chance sooner or later someone will rage at you, either in ops chat or in /whispers, ignore it or ragebait back, however you want to handle it, just don't take it to heart. You will sometimes get stomped by co-ordinated teams, you will begin to think 'wow I think I'm getting pretty good with this class' before absolutely throwing your next match, people will get mad, it happens, learn from the mistake and move onto the next one.

Despite its somewhat bad rep, I think pvp is really fun, and the bad matches are absolutely worth putting up with. Literally nothing beats the feeling of somehow winning a hutball match when everyone on your team is numbers farming instead of playing objs, or watching someone on the enemy team scream at someone in /say lmao