Assuming that you have the X Window system properly configured (more on this later), and that you're working at the command-line at one of the virtual consoles, you should be able to start X by simply typing:
This should, if all goes well, dump you into your default graphical environment. If this fails, chances are your X configuration is broken; proper X configuration is covered later in the documentation.
There are actually two variations for starting an X session:
$ startx start X in foreground $ startx & start X in background
The difference here is that starting X in the background leaves that virtual console free if you need to switch back to it later (this will be explained shortly). Simply, there's no real reason not to start X in the background, so you might as well make a habit of it now.