I take it we're talking about the coloured/transparent model;
This is a PPC G3, so I'll get the post moved to the correct forum in a moment.
These models didn't come with wifi as standard, indeed the earliest ones didn't even have a slot for a card.
I would normally suggest an Airport Express base station connected to the ethernet port to use as an access point, but the AEs currently available have to be set up with a version of Airport Utility which won't run on your model
Other choices are third party wi-fi bridges or USB wi-fi dongles (but finding a compatible one may be difficult)
There should be several threads on the subject (G3 wireless) so do a forum search, too.
Fair enough. Much the same still applies, but it may be possible, depending on particular model, to upgrade the OS so that AU would work. However as that is a pretty expensive way to go, you're probably best pursuing the other options first. I know some of those threads gave recommendations for specific USB dongles, and at least one for wi-fi bridge.
The pre-1Ghz iMacs you can add an original Airport card to, but it won't support anything better than 11 Mbps wireless with very weak WEP encryption. The 1 Ghz and newer PowerPC iMacs support the Airport Extreme card and 54 Mbps wireless and WPA2 encryption. For those with the older airport support, I recommend getting an ethernet wireless bridge with WPA2 802.11g support. For those with existing video streaming needs needing a fast wireless LAN faster than 54 Mbps, get a wireless bridge with 802.11n support. For all others getting an Airport Extreme card these days is difficult but not impossible. Also installing it in an iMac G4 is not child's play, as the RAM clips under the card are quite fragile. So be careful you both ground yourself electrically and install it carefully. 802.11a wireless support doesn't exist with the Apple Airport cards for PowerPC Macs.