AirPort is supported by all slot-loading iMac G3 models -- those with 350 MHz and faster processors -- except for the iMac G3/350 (Summer 2000 - Indigo) which does not have an internal AirPort slot. No iMac G3 models had AirPort installed by default.

Installing an AirPort card in a compatible iMac G3 model is quite simple and nearly the same procedure as upgrading the RAM in the slot-loading iMac G3 models. Effectively, you turn off the computer, place it "face down" on a soft surface, twist a coin in the small door to "unlock" access to the AirPort slot (and memory banks), attach the antenna, and plug in the card. Apple provides step-by-step instructions on the Apple Support Site.

However, as aftermarket AirPort and AirPort Extreme cards are expensive, you may also wish to consider a third-party USB wireless device in lieu of the internal card if one has not already been installed and the iMac G3 is running a later version of MacOS X.

http://support.apple.com/kb/HT2367