which GUI are you in, or console?
what kind of ethernet card?
LAN with static IP or DHCP?
off the top of my head, check the menu if your in KDE as root and look for a system admin menu and go from there, should be a networking choice someplace, comes standard with KDE, if not, the old fashioned way is easy and straight to the point
in console mode as root type:
lspci---> to see if card is recognized by kernel
lsmod----->is driver loaded yet?if not do next line
modprobe <driver>
ifconfig eth0 <static IP address> -net <netmask> up, if that syntax fails do "man ifconfig" and check man page
then add appropriate nameserver lines to /etc/resolv.conf file with a text editor
then do: route add -net <subnet_IP> and then do:route add default gw <ip of gateway>
That will get you all set for a static IP, for DHCP ive no clue for mandrake, before going thru all the above, try typing "net" in a console and then the tab key 2 or 3 times and see what pops up, the autocomplete might show something like "netconfig", use that instead
tell me what kind of card you have and I'll find the driver name for you, they are abbreviated, you can find the list in the /etc/rc.d/rc.modules file if you have too, probably a list in the kernel files too........ive been doing it long enough I just usually know off the top of my head, they usually do it like this:a 3com509 card would be 3c509, but a realtek8139c would be rtl8139too, its a little weird sometimes