The eMac (I think all of them) has a slot for an airport card. It's located behind a protective piece of plastic behind the optical drive door, just above the drive itself.
It wasn't a very common option, to my knowledge, so I'll assume you don't already have one.
The good news is, all you need to do is get your hands on an airport card and stick it in, and you've got B wireless (slow, but if it's just for internet use it's fine). The bad news is those cards are VERY in demand and no longer manufactured, so they tend to sell for a premium. If you're lucky you should be able to get one for about $50.
Alternately, you could use either a "bridge" or a router running in bridge mode (essentially a reverse wireless hub--has an antenna and one or more Ethernet ports, but the signal goes from the wireless connection to the wired instead of the other way around). These might actually end up costing more than an Airport card, depending on the brand and model.
There are USB dongles as well, but I don't know how confident I'd be in those working on a Mac.