All you need to do is go to System Preferences (located in the Applications folder) and click on Network. Select your Ethernet connection and set it to DHCP.
BTW, are you also providing DNS information from the DHCP server? If not, then you're going to have to manually input them on the Mac.
As for 10.1.5, I would recommend purchasing a copy of Panther at the very least. You're going to need about 512 MB to make it a pleasant experience on a G3 iBook. Also, most apps you'll find on the Internet have made the cutoff at 10.2, since that version was really the one that was "ready for prime time." 10.1.5 was better than 10.0.x, but 10.2 and up were a major improvement over earlier versions of OS X.
Check the following link for good prices on OS X if you're still interested in purchasing: