How do I start X?

Surprisingly, startx will do the trick. man startx for more information.

Of course, many people would rather not have to type startx every time they boot up their machine. Another program, called xdm, is available (and probably already installed) to relieve your pain. xdm can be started on bootup by changing the default run level from 3 to 5. man inittab for specifics on this.

The exception to this is Debian Linux, or if you are Richard M. Stallman, Debian GNU/Linux. That's a political debate that isn't likely to go away any time soon. Debian doesn't do anything the ``normal'' way. In the case of debian, you need to edit a configuation file /etc/X11/xdm.options.