If you haven't installed your network card and either re-configured the network subsystem or recompiled your kernel with the appropriate adapter driver, do so now. Descriptions to perform this task is beyond the scope of this document but are covered in the Networking HOWTO.
Install TCP/IP networking, such as the net-tools package, if you don't have it already.
Set IPADDR to 192.168.0.x (1 < x < 255), then set NETMASK to 255.255.255.0, GATEWAY to 192.168.0.1, and BROADCAST to 192.168.0.255.
Redhat (Mandrake / TurboLinux / etc): You can edit the /etc/sysconfig/network-scripts/ifcfg-eth0 file, or simply do so through the Control Panel (Linuxconf).
Slackware: You need to edit the /etc/rc.d/rc.inet1 file to configure the network subsystem.
To Add: Debian, Suse, Caldera, etc. Please email email@example.com if you can tell me what distro uses what files to configure the networking subsystem.
Beyond this, most Linux distributions use significantly different network configuration mechanisms let alone other UNIXes such as SunOS, BSDi, Solaris, AIX, TruUnix, FreeBSD, etc.). Please refer to your specific UNIX documentation for more details.
Add your domain name service (DNS) and domain search suffix in /etc/resolv.conf and for the appropreiate UNIX versions, edit the /etc/nsswitch.conf file to enable DNS services.
You may also want to update your /etc/networks file depending on your version of UNIX and the system's settings.
Restart the appropriate services, or simply restart your system.
As an initial test, run the ping command: ping 192.168.0.1 to test the connection to your gateway machine. (This is only an INTERNAL LAN connection test, so you might not be able to ping the outside world yet.) If you don't see "replies" to your PINGs, please verify your network configuration.