Well like with any game in general the first thing you should do is to learn what your character can do. This is a bit more difficult in fighting games since you're expected to switch between multiple characters, but nonetheless you should take some time to check out your character's moves and learn what they can do for combos, anti airs, air to airs and whatnot. After that comes the tricky part, which is movement. If you want to git gud at a fighting game you need to put your metaphorical experience points into your movement. Zoning, footsies and the like are all absolutely necessary for high level play. And of course I can't forget the addition of mind games. Fighting games are like martial arts, every move has a counter, and with things like baiting, conditioning and even just simple mixups can be used to make your opponent do the moves and movement you wan them to, so you can counter those moves easily and efficiently.