Up arrow turns on the main thruster. Left and right arrows turn on the directional thrusters. Zoom in and out with "i" and "o". Speed up time with "s"; slow down time with "d". Switch the destination to Earth with "e"; switch back to the Moon with "m". In the lower-left is a "radar screen", which shows you where you are relative to the planet, as well as the direction of your current velocity vector. This radar becomes very important when you get lost in space! d is your current distance from the planet. v is your current velocity. "orbital v" is the theoretical velocity you would need to obtain a stable circular orbit at that distance. At the top, it shows the width of the screen in thousands of km.
This game simulates Newtonian mechanics. In playing the game, you learn how to get a spacecraft into orbit. Try getting into a low orbit, then get back into a high orbit. Try elliptical orbits, too. I've found it's really hard to get it into a tight orbit. Also fun is to get it into a really slow orbit. Like, 10 minutes per orbit. Come back after an hour or so, and see if it's still in that orbit. No, really - it's fun! Another thing to try is the "slingshot effect". This involves going in very close to the center of gravity of the "planet" (which is actually modeled as a black hole). It's possible to get thrown a million pixels out into space! Then, try to navigate back home, and get into orbit again. Once you've mastered the Moon, take a journey to Earth! Pressing "e" causes the Earth to become the destination. (Press "m" to make the Moon the destination again). You'll need to use the radar for a long time, before you get anywhere near the Earth. You'll also need to zoom way, way out (press "o" to zoom out; press "i" to zoom in). If you get impatient, you may find it useful to speed up time: press "s" to double the rate of the passage of time. To slow things down, press press "d" to halve it.
