First of all, you probably need a cross-over cable, not a straight-through ethernet cable. Look at the back of the PC and if the little green light beside the ethernet jack is not on when the cable is connected between the Mac and the PC, you have the wrong kind of cable.
Once you get the right kind of cable, you have set the Mac's TCP/IP control panel to:
Connect Using Ethernet
Configure Using DHCP
Then you have to set up Internet Connection Sharing on the PC following the instructions
HERE