That’s a bit of a broad question. I’d say the 3 biggest things are:
Don’t Quit: Against more experienced players, you’ll start out losing a lot. A LOT. Try to learn from your mistakes. Heck, ask them what you’re doing wrong and what kinds of mistakes you’re making so you don’t repeat them next time.
Offline >>> Online. “Don’t take online too seriously” could also work here. Since fighting games are usually precise affairs, where even a slightest fraction of a second could be the moment where you either grabbed victory, or fell into defeat, network lag does not do anything for the experience. Also, there are many tactics and strategies that people utilize online that are ineffective or outright suicidal if tried offline. If at all possible, try to play offline against people rather than online, even if you have to go a bit out of your way. It’s the way the games were meant to be played. Oh, if you get sent hatemail for beating someone really badly, you’re doing something right.
Don’t count your previous game experience towards fighting games. Fighting games are an entirely different breed apart from shooters or RTS games. What fighting games do reward however, is being able to analyze, react, and adapt quickly and well. Most of all, they reward practice, and by practice I don’t just mean practicing combos against the training dummy, but playing against other players constantly.