Welcome to LQ!!
You have to give more details about your system: linux distribution, wired or wireless, modem or router and so on. For example for a wired connection you have to be sure the network card is correctly recognized by the system and it receives an IP address via DHCP, then you have to configure the pppoe service (Point to Point Protocol over Ethernet). It can be done in different ways depending on the distribution.
Post the output of the following commands if you don't mind: