on newer systems are normally built into the motherboard and the port....looks like a larger version of a regular modem 'hole'......is usually located near the USB ports on the back of the computer. If you don't have a LAN built into the motherboard, and IF your DSL kit doesn't include one, you will have to purchase a LAN/Ethernet card and put into one of the PCI slots on the motherboard and then install the drivers for it that come with the card.
You can actually go into Device Manager, click the Plus in front of Networking Adapters and see if there is one listed. If so, you're in business because it's on your motherboard and already installed. However, if you find the port on the back and Device Mgr doesn't show it, you may have to enter your bios during a boot and actually enable it so Windows can find it and normally can install it from its own database of drivers if you can't locate the install disk for your motherboard.
You can call your DSL company and ask if they supply one with the kit and if not, you can get one from Staples or Circuit City, etc. for about $20 or so. You don't have to have the biggest or baddest on the shelf to make it work easily with DSL....an inexpensive 10/100 card will do nicely.
TONI