We've covered some of the essential facts about what you can put into a LATEX document, but we still need to look at how the document must be organized as a whole.
Every LATEX document should begin (except possibly for
some comments) with a \documentclass
command. This command has a parameter that specifies
the type of document: article, report, letter, or book.
The command can also specify
some options for the document. The basic font size
can be specified with the 10pt, 11pt, or 12pt options.
The default is 10pt. Specify 11pt or 12pt for larger-sized
characters. The twocolumn option will cause text to
be set in a double-column format. Here is a
typical example:
\documentclass[11pt,twocolumn]{article}
Following the \documentclass command comes
the preamble, which can contain commands that
influence the contents or appearance of the
document. The preamble does not itself produce any
output. The preamble often starts with some
\usepackage commands. A package makes available
a set of extra, non-standard commands for use in the document.
For example, \usepackage{amsfonts} and
\usepackage{amssymb} make a large number of
extra mathematical symbols available. (In fact, the
\therefor command, which was listed in a table
earlier in this paper, requires these packages.)
The preamble command \usepackage[dvips]{graphics}
makes it possible to include graphics, as discussed
later in this paper.
Commands that specify the title, author, and date of the paper are often given in the preamble. For example:
\title{A Short Introduction to \LaTeX}
\author{David J. Eck}
\date{October 22, 2003}
The information in these commands is actually used later,
in the body of the paper, when you give the
\maketitle command.
Page layout in a document is partially controlled by
the value assigned to various lengths, such as
the size of the margins and the distance from one line
of text to the next. The preamble can contain
commands for customizing some of these lengths
using the \setlength command. For example,
for this paper, I use the commands
\setlength{\topmargin}{0in}
\setlength{\textheight}{8in}
to increase the vertical size of the text on
the page from its default value. The first parameter is
the name of the length that is being set. The second
parameter is the value, which must be give in units such
as in, cm, or pt. The distance between lines of text--the
so-called baselineskip--is handled a little
differently. You have to specify a factor by which
the default baselineskip is multiplied. A factor of
2 will give double-spaced text, although a factor of
about 1.8 probably looks better. The command
that you would use in the preamble for double-spacing is:
\renewcommand{\baselinestretch}{1.8}.
You can actually use this command anywhere in
your document to change the baselineskip for
the following paragraphs.
LATEX is actually a programming language, in which you can
define your own commands. The preamble often includes
definitions of new commands that will be used in the
paper. However, this aspect of LATEX will not be
discussed here.
After the preamble comes the document content. The body
of the document begins with \begin{document}
and ends with \end{document}. The actual content
of the document is typed between these commands.
A document of any length is typically divided into
parts, chapters, sections, subsections, and subsubsections. In
LATEX, an article can have only sections, subsections,
and subsubsections, while books and reports can
also have parts and chapters. To start a new
section, just say \section{title} where
title is the title of the section. The
same thing works for parts, chapters, subsections,
and subsubsections. You can use \\ in
a title to force a line break.
Sections, subsections, and so on will be automatically
numbered for you by LATEX. If you don't want the
numbering, you can add a 
after the name of the
command: \section*{title}. This will suppress
the numbering.
If you include the command \pagestyle{headings}
in the preamble of the document, LATEX will add running
headings to the top of each page. The headings will
contain the section or chapter titles. Sometimes, the
heading will be too long to fit. In that case, you
can provide a short title for the headings. The
short title is given as an option to the command:
\section[short-title]{long-title}.