Latex
The standard way to typeset mathematical documents is using LaTeX.
LaTeX requires some time and practice to get used to, but in the long
run it's well worth the trouble.
If your computer runs Linux, then Latex is already installed, and you
probably in any case have no need for further instructions.
If your computer runs Mac OS, then you should start by downloading and
installing the MacTeX package. One you have finished the installation
open the application TeXShop. Download the sample Latex document
below, open it from within Texshop, and click the 'typeset' button in
the upper left corner of the Texshop window (you may have to click
this twice because Latex has an automatic equation numbering and
cross-referencing system that requires multiple passes to work
properly). If everything is installed correctly, a PDF version of the
Latex'ed file should appear. This should get you started.
If you use Windows then you'll want to download the proTeXt package
and follow its (slightly complicated) installation
instructions. (Caveat: I have never done this myself.)
Here is a link to the sample document in pdf format:
Sample Document (pdf)
Link to pdf
Here is a link to the sourxe file usee to create the pdf of the sample document:
Sample Document (source file)
Link to source file
Here are links to some useful documentation:
The Not-So-Short Introduction to Latex
Link to pdf
The Wikibooks Latex Guide
Link to pdf
The Comprehensive Latex Symbols List
Link to pdf