What is LaTex
The usual word processors are not powerful or flexible enough for writing scientific papers. LaTex is the system for scientific writing.
A document has appearance and structure. The appearance is the content that you see. This is similar to the web pages you see through the browser. The structure is the components such as section, paragraph, theorem, exercises, etc. This is similar to the html code for the web pages. What the browser (such as Explorer or Safari) does is to interpret the structures into the visual appearance. The interpretation follows a style sheet that says something like theorems should be headed by Theorem, and exercises should be headed by Exercise. If you change the style sheet, then the design of your document changes, but the content remains the same.
An installation of LaTex includes two parts. The first is the basic engine that interprets the structure into the visual document. This part is buried into your computer, that you never see. For Windows, you may install MiKTeX. For Mac, you may install MacTeX.
Write in LaTeX
A LaTeX document is very similar to the web code. It starts with some basic parameters for the document. Then you can write the main part, which is the content of the document. The best way to prepare such a document is to start with a sample document.
After you write the LaTeX code, you need to typeset the code to produce the document.