All instances in GameMaker: Studio have certain "built
in" properties that you can use and set to govern how they look and
behave. Direction is one of those properties and can be
used to set the direction of movement of the instance when the
instance has a speed other than
0. Note that directions in GameMaker: Studio are usually
calculated as 0° being right, 90° being up, 180° being left and
270° being down.
if keyboard_check(vk_left) direction += 5;
if keyboard_check(vk_right) direction -= 5;
The above code will change the direction of movement of the instance based on which key (left or right) is pressed.