Welcome!
All USB wireless adaptors require drivers. Most do not have Mac drivers available. Check the manufacturers' web sites carefully to see if they have proper drivers.
The most Mac-successful adaptors based on reports here come from this vendor that is a Mac specialty house:
http://eshop.macsales.com/search/wireless+USB
The ones they sell have proper drivers. However, some models require Mac OS 10.5 and early eMacs can't support higher than 10.4.11.
The are two eMac families with two variants each. Knowing which on will help us with future questions. Please use this Apple article to find out which eMac you have and post the results (but don't tell us your serial number):
http://support.apple.com/kb/HT2395
That will make our helping you easier and faster