If you’re on a network with *nix or *BSD machines and need to access programs running on those machines from your BeOS machine, you’ll find everything you need to know about installing and running X services for BeOS in this article by Daniel Berlin, written for the former Be Magazine.
Once you’ve got things set up, you’ll find that the de facto standard program to startup X services, xinit, does not function properly on BeOS at the present. This in turn leaves users having to start up each service (ie. X, an xterm, a window manager, etc.) manually. There is however a simple workaround. Create a script that looks something like this:

Xbeos -ac &
sleep 1
xterm &
twm &

and name it xinit. Now make the script executable by typing

chmod 755 xinit

Drop the script into /boot/home/config/bin. The next time you want to start X, just type xinit, and you’ll have X, plus all the goodies pop up in one shot.