You can find complete LATEX systems for your own particular operating system here.
In the Unix and Macintosh worlds, TeXLive is the most common LATEX distribution; the older teTeX distribution is still functional but deprecated. In the Windows world, the current recommendation is the protext distribution, which installs miktex and a whole host of useful auxiliary applications.
You can use any editor to write LATEX code. A list of editors that many people have found useful can be found at http://en.wikipedia.org/wiki/LaTeX in the ``Front Ends'' section. Some of our favorites include: