Your input file must contain the following commands as a minimum:
\documentclass{class}
\begin{document}
... your text goes here ...
\end{document}
where the class selected is one of the valid classes for LaTeX.
See section Document Classes (and see section Overview of LaTeX and Local Guide), for details of the
various document classes available locally.
You may include other LaTeX commands between the \documentclass
and the \begin{document} commands (i.e., in the `preamble').