sturm is correct, you just need a router. And it definitely is the preferred way, by far.
But just for completeness, you could also use ICS (assuming you are running Windows). This turns your PC into a gateway (the functional equivalent of a router) for the other PC. But it will require that your PC have two network adapters; one for the connection from the gateway PC to the modem (which you already have), and the other from the gateway PC to the other PC. Then you just follow the directions for setting up ICS.http://support.microsoft.com/kb/306126
Again, sturm has identified what most ppl should do. A router is more secure, easier to manage, supports multiple PCs (not just that one additional PC), doesn't require you keep the gateway PC running whenever the other computer needs Internet access, etc. But in a pinch, or if you have a very limited need, and your PC has at least a second network adapter, it may be a passable alternative.