Create a bullet sprite - Pay attention to the size! It only needs to be small.
Create a bullet object and name it "obj_bullet".
On the player object add a "Global Left Pressed" event under the Mouse menu and add the following code.
Next, add a Create event within the bullet object and add the following code to set the direction and speed of the bullet.
To stop bullets going through walls, add the following as a collision event on your bullet object
To destroy the enemy when hit by a bullet add the following as a collision with the bullet on the enemy object.