You can experiment however you like here; if you don't know what to do, here's an objective for you: reach a stable circular orbit around the moon
Simulation controls:
Mouse - zoom and pan
Left/right arrows - Rotate ship
Up/down arrows - Increase/decrease throttle
Q/E - Zero/full throttle
P, R, I, O, N - Point ship prograde, retrograde, radial in, radial out, and manual controls
, and . - Slow down and speed up time
H - Reset