The file doc/guide.sgml is SGML source for the User's Guide. As you probably don't know how to format it yet, there is formatted plain ASCII in doc/guide.txt. There's also guide.dvi and guide.ps, if you wish to print them instead. The first thing you should read is doc/guide.txt, which explains how to install the software, and how to start writing documents with it. The file doc/example.sgml is a simple example document which you can use as a tutorial.
For real-life examples of uses of Linuxdoc-SGML, look at the Linux HOWTOs on sunsite.unc.edu:/pub/Linux/docs/HOWTO/other-formats/sgml. These HOWTOs are written in SGML and automatically translated into PostScript, dvi, and ASCII using this system. You can also look at the URL http://sunsite.unc.edu/mdw/HOWTO/ which has HTML formats of many Linux HOWTOs, produced using Linuxdoc-SGML.