Well, it depends on a number of things - does your iMac have an Airport or Airport Express card installed? If so, you're going to need a basestation which can be picked up for around $50-100USD (802.11g models are the most common now) and a crossover cable.
I use a Netgear WGR614 with my Powerbook without any hassles.
After you've got your router/basestation, configure the Airport using the Network System Preferences pane in OS X to use the basestations IP as the 'Router'.
Configuring WEP and MAC address filtering is a little more of a hassle, but using the online help provided with the router it should be self explanatory.