The easiest thing to do would be to replace your existing router with a wireless one. Wireless routers generally have four wired ports, too.

If you want to use a wireless router *and* your existing router, you'd be using the wireless router as an access point. To do that, disable the wireless router's DHCP server and connect one of its LAN ports to a LAN port on your existing router using a crossover cable. (If either router has autosensing MDI/MDI-X ports, you could use a standard patch cable or a crossover cable.) Or to do it the easy way, just buy an access point instead of a router.