Dialup:
If the powerbook has an internal modem then you can use dial up. Otherwise you could get an external serial modem and a mac serial to rs232 cable.
Ethernet:
If you can track down a SCSI to Ethernet adapter or a LocalTalk to Ethernet adapter with the right drivers then you can connect to a ethernet network. Then you would need a home router between the cable modem and the powerbook.
I had my old 145b Powerbook online on Prodigy for a while in 1995/96. Dial up. Also fax modem thingie. Can't remember what it all was and how it was hooked up; but i WAS online.