I am not a lawyer, and I have not been trained to be a lawyer, so here is my nonexpert opinion. If you are serious about selling your computer game at retail stores, find an established computer game publisher; make certain to retain the copyright to the product (or, at least, co-ownership of the copyright) and to take a fair cut of the profits. If you want to sell your game as shareware, you could look for premade, boilerplate end-user license agreements (EULA's) for shareware developers or consult a lawyer (the more expensive route). If you "copyleft" your computer game, you retain the copyright; however, you have basically waived most of the rights that go with that to the general public.

Of course, you could always try to write your own end-user license agreement, but you would have to make sure to specifiy all the terms: the warranty (if any), legal disclaimers, end-user usage rights and duties (restrictions), etc. Going this route, unfortunately, may lead to an EULA that is unenforceable and invalid or that contains loopholes.