The first line of the input file should normally
consist of an appropriate \documentclass command.
If an article (or similar document) is to be produced
on A4 paper, and if the main body of the text is to be
set with a font whose natural size is `12 point',
then the appropriate \documentclass command is
\documentclass[a4paper,12pt]{article}
Other forms of the \documentclass command can be used
for letters, reports or books. If 12pt is omitted from
the \documentclass command (with the preceding comma),
then the document will be set in a `10 point' size. One may
also replace 12pt with 11pt.
The documentclass command may be followed by certain
other optional commands, such as the \pagestyle command.
It is not necessary to find out about these commands when first
learning to use LaTeX.
After the \documentclass command and these other
optional commands, we place the command
\begin{document}
This command is then followed by the main body of the text, in the format prescribed by the rules of LaTeX.
Finally, we end the input file with a line containing the command
\end{document}