Go under the Apple in the upper left hand corner to "about this mac", more info and see if the air is even seen. If not then first check its connectors, that failing then replace it.
Here's how to do it: iBook G4 14" 1.42 GHz AirPort Extreme Card Replacement
Here's the part: iBook G4 12" 1.33 GHz or 14" 1.42 GHz Airport/Bluetooth Board
802.11g Airport wireless / Bluetooth 2.0 internal card — 4 available at 69.95 each.