Regardless of whether your motherboard has a built-in NIC (LAN card) or you have a separate PCI card, you need a functioning NIC in addition to an ADSL modem.
A network cable runs between your NIC and your ADSL modem, and a telephone cable runs between your ADSL modem and the wall. Some ISPs rent an ADSL modem with their monthly package, but if yours doesn't you should check with the ISP tech support to see what kind of ADSL modem you need to get.