So you've been sent a TeX file: what are you going to do with it? Well, the good news is that TeX systems are available, free, for most sorts of computer; the bad news is that you need a pretty complete TeX system even to read a single file, and complete TeX systems are pretty large.
TeX is a typesetting system that arose from a publishing project (see "what is TeX"), and its basic source is available free from its author. However, at its root, it is just a typesetting engine: even to view or to print the typeset output, you will need anciliary programs. In short, you need a TeX distribution - a collection of TeX-related programs tailored to your operating system: for details of the sorts of things that are available, see "TeX distributions" or "commercial TeX distributions".
But beware - TeX makes no attempt to look like the sort of WYSIWYG system you're probably used to (see "why is TeX not WYSIWYG"): while many modern versions of TeX have a compile-view cycle that rivals the best commercial word processors in its responsiveness, what you type is usually markup, which typically defines a logical (rather than a visual) view of what you want typeset.
However, in this context markup proves to be a blessing in disguise: a good proportion of most TeX documents is immediately readable in an ordinary text editor. So, while you need to install a considerable system to attain the full benefits of the TeX document that you were sent, the chances are you can understand quite a bit of it with nothing more than the ordinary tools you already have on your computer.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=readtex