A debian base system is now installed? It should have asked for your network settings at the installation phase. If you have skipped those, you can set them manually or automaticly
by installing a package called etherconf (that is: 'apt-get install etherconf', but you need either a debian cd that has the program or a configured network (ironic in a sense)).
To do it manually, you have to a) find out what network card do you have and load a proper module for it (the 'lspci' command helps) and b) bring the network interface up.
a) Once you have figured out (with the help of "lspci") what your network card is, you can check that is the driver part of your kernel (run something like "dmesg | grep eth" and check if the output mentions something about your network card). If so, move to section (b).
If the network card driver is not in the kernel, you should run a program called "modconf" and find a proper module from the list (if you guess wrong, it most probably says something about that you don't have a hardware for this driver and you have lost few seconds of your life; not very fatal).
Actually this phase can be automated by installing a package called "kudzu" or "discover". Then they just announce that you have this and this hardware and i would like to install this driver.
b) Put following lines to /etc/network/interfaces
iface eth0 inet dhcp
or if you do not obtain your ip-address automaticly by dhcp, put something like
iface eth0 inet static
(with your values, of course).
and run "ifup eth0" and you should be in the network.