There's not much TO know. Sign up, you start with 400 salty dollars, bet on fights and try to make money. Scamming is when a character gets a health lead and then becomes unable to be or do damage, effectively ending the time until the fight ends (70 seconds usually, at the end if neither person is dead "Thanks Obama" plays and whoever has more health wins. If they're even it's a draw.). Frequently characters that do this wind up scamming themselves because they use whatever their scam skill is when they're down on health, and chat will freak out about self-scamming. It's usually quite a sight to see, and you'll learn to recognize the characters prone to it. One of my favorites is (I think it is) Johnny Cage, who has not 1, but 2 scams. One where he punches a person so hard the go up and don't come back down, the other where he just punches them into a coma.
Other terms/stuff to know: A potato is a character who has little to no AI. Matches between 2 potatos are probably the most boring thing you'll see, and they go on FOREVER because they never attack so it always Obamas out.
Googoos (Not to be confused with Jojos) are characters made by one particularly prolific creator who... has eclectic tastes, and doesn't animate worth shit, and tends to make really strong characters. You'll know if a character is a googoo or not if it has an awkwardly out of price sprite, and just skips from their normal frame to an attack frame and does a ton of damage.
Mexi____ is a term for when something has really absurd numbers, like mexihealth where they just don't die or mexibeams which do absurd damage. The term comes from the fact that a lot of characters, usually characters from DBZ, were made from people form Brazil who couldn't write AI to save their lives so instead they just gave characters absurd damage and health and the like, and "All mexicans are the same" so instead of Br___ it's mexi___. Yes it's a crude joke, yes it's still mildly entertaining.
Also, the chat is 90% of the fun. If you can't read it/participate in it, you're missing out.
Ninjad but meh