There are a handful of major Linux distributions. For information about them, and how they are installed, see Matthew Welsh's Installation and Getting Started, or IGS for short. It's located at the Linux Documentation Project Home Page, http://www.linuxdoc.org/, and on the Linux FAQ home page,
The information in IGS is somewhat dated now. More up-to-date information about first-time Linux installation is located in the LDP's Installation HOWTO, also located at the LDP Home Page.
Postings on the Usenet News groups, including the FAQ, are archived on http://groups.google.com/. Search for "comp.os.linux.*," "alt.uu.comp.os.linux.*, or whatever is appropriate, to retrieve articles from the Linux News groups. ("What News Groups Are There for Linux?")
Commercial distributions are available from book and electronics stores. Some hardware vendors now ship systems with Linux pre-installed.
There is a very thorough installation guide on line at http://heather.cs.ucdavis.edu/~matloff/linux.html.
Some distributions can still be installed via anonymous FTP from various Linux archive sites, but in many cases, the size of the distribution makes this impractical. ("Where Are the Linux FTP Archives?") There are also a large number of releases which are distributed less globally that suit special local and national needs. Many of them are archived at ftp://ftp.tux.org/