How can I connect two computers using a ethernet cable

You get one of those little plastic things the you can plug 3 cables into it. One that goes to the router or internet source and the 2 other ones go to the computers. This answer may be wrong if you mean connecting the computers to each other.

Hello
You can connect 2 computers together with one 'crossover cable'.
A crossover cable is an ethernet cable with standard rj45 connectors at each end but the cables are wired differentlty than a regular ethernet cable.
If you want to connect 2 computers directly together by their NIC's it has to be a crossover cable.
Another way to do this is by connecting each computer to an ethernet switch or hub and this can be done with regular ethernet rj45 patch cable.
If you need to get a crossover cable radioshack sell them.
Regards
C