I came to this forum in search of help, too, so take my humble advice with a grain of salt, so to speak.
I'm looking at OSX 10.3.6; if you have an earlier version, things may be a bit different, but not much. You must have an administrator's login name and password to set this up, or make changes.
If the iMac you're describing uses Mac OSX, I think you'll be pleasantly surprised. Out of the box, OSX is set up to assume that DHCP should be activated. If the iMac's already been set up before, though, the network settings may have been changed.
On the Mac, click on the leftmost menu, the "Apple" menu [as it's called]. Choose "System Preferences".
A window pops up that is pretty much like the Control Panel window in Windows, and one of the icons there is called "Network". Double-click "Network".
There are two pull-down menus, "Location" and "Show", at the top of the Network control panel that will pop up.
For "Location", choose "Automatic". For "Show", choose "Built-in Ethernet", since that's how your iMac will connect directly to the router.
Beneath all this are 5 "tabs", I guess you'd call them, or at least a horizontal row of buttons. The leftmost is labeled "TCP/IP". If that's not highlighted already, click it once.
There will be a pulldown menu labeled "Configure IPv4". Choose "Using DHCP".
It might not hurt to put a unique name in the field labeled "DHCP Client ID", but try without it, first.
That's probably all you need to do. If that doesn't get you onto the network, hopefully someone else can take it from here.
I've never set up an OSX machine for a wireless home network, but I set them up for an office LAN frequently, and that's all I need to do because my company uses DHCP.
Hope this helped,