LaTeX Online. This works in a browser window -- so to try this
you don't install any software on your own computer. You type in your
own text in the left window -- or just modify the sample there). Then
press "Result Preview" at the top of the left window. The output
appears in the right window (after you scroll-down a bit).
TeX References: Near the bottom of this TeX
References page there are links to tutorials to get started and
the TeX Cookbook
TeX Software. The basic part is TeX and LaTeX.
You can use it with any simple editor -- or even a word processor such
as MS Word (but then save your work as a plain text document). Most
people prefer to also use a "front-end" to TeX/LaTeX that has an
editor and various ions adapted to TeX.
Mac: TeX Live + TeX Shop = Mac TeX:
TeX Live is the main part and TeX Shop is a good front-end.
You can just use this and any editor. See below for a link to list of
Linux: each Linux distribution has a standard TeX/LaTeX package (this
is what I use with a very primitive font-end.)