You can write the LaTeX source using any text editor (like MS Notepad) or word processor (like MS Word). Just make sure to save your file as "Plain Text".
More convenient is the free program texnicenter, a text editor specifically designed to work with LaTeX. If you're willing to spend the money (about $30) you can download a nifty word processing program called WinEdt designed to work with your LaTeX installation.

For macintosh, look here.

To try LaTeX before you get your installation up and running, or if you don't have access to a machine on which you can install this software, you can use this On-line LaTeX compiler.

Help with the LaTeX commands can be found here. Here's a list of commands for producing math symbols.

Although the ubiquity of Adobe Acrobat reader has made the PDF file format a popular option, LaTeX documents have historically be typeset in the Adobe Postscript format. A powerful postscipt viewer program is GSView. To use it, you need to download both Ghostscript and GSView.