Afraid to get into Online

I got FighterZ for Xbox One, and so far, I like the game. That said, I'm afraid to get into online. Why's that? Well, afraid that my first opponents are players who are so, so super adept at the game even they're at the rookie ranks that there will be no chance for me to fight back. Speaking on how my experience with Ultimate Marvel vs. Capcom 3 went when playing it online.

With that said, which characters should I start learning to take online, and which Z-Assists are the best bet for a rookie like me? I should say, I know how to do combos, but I'm aware that my combos won't be enough. How do I extend combos without going all mad button mashing? One thing I should mention is that I don't have all the DLC characters, so I have the Base Roster, and the Season 1 DLC.

Any tip is appreciated. :)