The button command should be used with a logic gate accompanied by button constants. You can find all button constants in BUTTON's help entry, but they are pretty simple to remember. They are... #A, #B, #X, #Y, #L, #R, #UP, #DOWN, #LEFT, #RIGHT. If you have a New 3DS you can also use #ZL and #ZR. After this, you can use a simple conditional to determine if a button is being pressed.