Could be a number of things, but the first things to try are Software Update for your iBook and firmware update for your Airport Base Station. Doing both those will ensure you have the latest bug fixes and after installing, both will restart, too.
Use Software Update on your iBook to get it up to at least 10.3.9. The 10.3.9 update includes fixes for WEP and WPA connections (if you use encryption for your Wi-Fi, and you should). And then look for an Airport update in the Software Update window, too. That will also fix some bugs and known connection problems.
Then, use Airport Utility or Airport Administration Utility (in your Applications/Utilities folder) to check your Airport. Login to the base station; you may get a message saying there is new firmware for your Airport. if not, use “Check for updates” under the “Airport Utility” menu item to check for new firmware.
Go ahead run the updates, because those updates includes bug fixes that will help your iBook connect. Your base station will restart itself after the update.
One other possible part of the problem: if you do have encryption set up, you’ll need to use WPA rather than WPA2, because the Airport card in a G3 iBook won’t work with the newer WPA2 protocols. So if you still can’t connect after your base station and iBook are updated, check your encryption setup in the base station setup with Airport Utility or Administration Utility.
And realize that since your iBook uses the older “B” wireless mode, and if you have a newer Airport Base Station or Time Capsule that runs with the newer “N” mode, your iBook will slow down every device on the Airport. So if you see a dramatic drop off in speed with your MacBook or other newer Mac, it’s because of the iBook.