With, for example FTTC, where a modem is provided for you, you would usually plug in a router and configure the router with your username and password. However, you can use a computer instead of a router, and get online by just plugging the network port of your computer in to the modem and set up a 'PPPoE' connection...

General Settings

PPPoE Service Name = Blank
PPPoE username = as supplied, e.g. x@a.1
PPPoE password = as supplied

Windows

Open the Connect to the Internet wizard by clicking the Start button Picture of the Start button, clicking Control Panel, clicking Network and Internet, clicking Network and Sharing Center, clicking Set up a connection or network, and then clicking Connect to the Internet.
On the How do you want to connect? page, select Broadband (PPPoE).
Continue to follow the steps in the wizard.

XP
Vista/7/8

Mac

See: Apple

Ubuntu

See: Ubuntu