I'm a game developer, and I'm working a space game, where I have to simulate a spacecraft's orbit, traveling from the earth to the moon. I would need the apoapsis and periapsis of the orbit. I have the mass, position, and velocity of the objects.
Only problem is, I absolutely don't understand astrophysics, so I couldn't understand the equations I found on the internet. So I would be very grateful, if someone could explain me this.