Windows 98 is really just as simple as anything else to setup. First question would be is the ethernet adapter on and recognized by the OS? Look in the control panel and "network" to see if you have a LAN connection. If you don't, get into the system and device manager to see if you need to install drivers for you LAN adapter. Also, do you have lights on the adapter when you plug the ethernet cable into it? No lights = problem.
Also, what are you hooking it up to? Are you plugging it into a Cable Modem (or DSL Modem), or a router? Though, really neither of these should make a difference these days cause the IP should be assigned automatically by default these are still things that could help us in the end.