A Dance Revolution Game system

I am trying to insert a combination of random keys (up, down, left, right to make it simple) and then when a monster hits the player, a combination of keys show up in the screen. The thing is, i don’t know how to make the arrows show up randomly in the screen.

Other of the problems too is how i can make only the first key of the combination available to be pressed, so it passes to the key that is next to the first key, making then this key available to be pressed. Is it through Booleans?