On a computer:
To make the character run to the right, press the D button (or the right arrow) on the keyboard.
To make the character run to the left, press the A button (or the left arrow).
To jump, press the space bar (or the up arrow).
On a smartphone or tablet:
To make the character run to the right, tap the right arrow at the bottom of the screen.
To make the character run to the left, tap the left arrow.
To jump, tap the up arrow.
To fire a shot (which becomes available at a certain level), press the fire icon button.