It is possible to "share" a printer which is directly connected to a computer. Some printers (and even some eclectic routers) can act as print servers, but in general your average home printer can be shared out via a computer. The only caveat is that the computer it is connect to (and of course the printer) must be on for the printer to be shared.
Instead of typing up instructions, I will instead refer you to an external article which includes step-by-step instructions (conveniently excerpted from "Home Networking for Dummies"):
http://tech.yahoo.com/gd/setting-up-...network/153491