Buy a Cable/DSL router (NOT ADSL router!). Plug the cable modem into the internet port of the router and then plug an ethernet cable from each computer to the router.
Run the setup wizard on the router, enable file and printer sharing on both computers and you should be up and running.
Both computers can then access eachother and the internet.
If you want to go wireless, get a wireless router but if the computers are near to eachother, I would save the money and hassle and buy a normal router.