Start by creating a subclass of Actor that will create the bullets (a Bullet class). The only thing a bullet does is move and disappears at an edge. It also has a speed state and a direction state that regulate its movement. Then you can work on having the shooter fire (create and direct) the bullets using a change in state of the space bar. I order to detect a change in the key's state, you will need to save the last state of the key to compare to the current state.