RedHat linux is probably the most easy-to-install Linux distribution
and if you have a good reason for using any other distribution it's
probably because you know much more about Linux than I do. If
you have no clue about what a ditribution is, just stick to
RedHat-5.0 (or the most recent you can find) and you can't go wrong.
You can buy the CD directly from RedHat http://www.redhat.com or a copy from, e.g., CheapBytes http://www.cheapbytes.com, in the latter case I reccomend to print the manual, downloading it from the RedHat FTP site or from any mirror.
You can even burn your own CD: in this case keep in mind that if you are not using a Linux or UN*X machine, you could have problems with the readibility of the CD, as the letter caps and/or the file permissions may not be correctly preserved on Windows or Apple boxes.