Fighters
1: Been playing fighting games since I was a young kid with a plastic box to stand up on at the arcades. Understanding what individual buttons do, the allocated spacing to perform then in, special/super attacks, and eventually what button combinations exist.
2: By losing multiple times, since majority of fighters originated from arcades- bosses were designed over powered (cheap) gimmicky to take more coins from you. Eventually you'll defeat the POS :]
3: Excessively jumping. In most (2D) fighting games jumping is taking a much larger risk allowing your opponent to have more options to block, avoid, or set-up a counter attack.
My tips for 2D fighting games
1: Start slow, work with your individual buttons (punches, kicks) use them in a match to win, avoid button-mashing.
2: Initially play the game with mindset you will most likely lose, when you lose a match you end up learning what didn't work in that instance.
3: Since most fighting games are 1v1, take your losses with full responsibility and do not make excuses for your loses.
4: Stick with learning 1 to 2 characters understanding their moves to the fullest.
5: Eventually play every character casually to get feel for them.
6: Study your opponents (if playing against another person) bad habits, punish them the next round they attempt it.