If you have a router connected to your DSL modem, things should be pretty simple. You can open up Mandrake's configuration tool (DrakConf I believe, but I don't use Mandrake), locate the network connection settinggs, and set your network card to get an IP address via DHCP. Assuming your router is set to hand out addresses via DHCP (and most are), you should get an IP address and have the other settings you need configured automatically.
If it doesn't work, please post more details on your configuration, such as the make/model of your router and Ethernet card. Also, which version of mandrive are you using?