Shoot for the moon means to 'aim high' try to achieve the best you can. Literally - the moon is so far away that you have to shoot to get to it.

Yvonne's answer is perfect. Let me tell you a saying. Shoot for the moon, even if you miss you'll still end up among the stars!

It means if you are capable enough to use a gun, you should find a powerful one to go across the atmosphere and hit the moon... now, the moon is gonna be pretty pissed but don't worry about that, she is used to recieve many many bullets nowdays... =p No, seriously... look at the moon as one of your dreams... it's high, huh? Now, even when you think (if you do) what you wish is unreacheable you still have to try. You decide to shoot the moon (as in working hard and take risks hoping for the best result there is ) and see what happens... (if your bullet hits a tree or a mountain, that doesn't prove that the moon or your dream were too far away... it just means now you know you have to change your ways but always keep trying . Because you wanted the moon, remember... not the star or the asteroid, THE MOON!)