What type of Internet connection you have (Modem, Cable, DSL, ISDN, T1)?
What your network hardware consists of (Direct connection, Hub, Router, USB)?
Internet service cannot share over the network as it is.
There are few general options:
1. Cable/DSL/Modem Router.
.
2. Software solution, Windows ICS, and the like.
3. Additional computer that acts as Internet Server-Firewall.