Start a match and do not block during the second or third rounds. Then, finish your opponent by pressing Back, Forward, Forward + Punch.
Brutalities
To perform brutality for any character, press Forward(3), Down, A.
Fatalities
To perform a fatality for any character, stand at any distance and press Forward(3), Down + Punch.
Fight as Ermac
Press B, C, B(2), D at the character selection screen.
Fight as Noob Saibot
Press D(2), C, D, B at the character selection screen. The screen will shake to confirm correct code entry.
Fight as Scorpion
Press Up, Left, then hold Right for approximately five seconds at the character selection screen.
One button fatalities
Enable the "Question mark menu" code. Turn on the quick finish option and press one of the following buttons when your prompted to finish your opponent.
A = Brutality
B = Fatality
C = Friendship
D = Babality
Question mark menu
Press Up, C, D, B at the options menu to display a question mark box, with quick finish, instant aggressor, and low damage options.