Well first you want to make sure your NIC is supported in Linux.
Try doing a ifconfig at the command line. If Mandrake installed it when you first installed it, then you should see it listed as eth0 then whatever after words. If it is there, you can then use linuxconf (or is it in Drakconf in Mandrake now, someone correct me if I am wrong) in the terminal window to get a graphical setup of all your network IP address, DNS and etc.
Its usually pretty easy setup if your NIC is installed already. If not, try the HowTo's at
http://www.linuxdoc.org under the main Ethernet HowTo to see how to get your ethernet card working. Let us know!