Your LaTeX2
input file must
contain the following commands as a minimum:
\documentclass{class}
\begin{document}
... your text goes here ...
\end{document}
or for LaTeX2.09 :
\documentstyle{class}
\begin{document}
... your text goes here ...
\end{document}
You may include other LaTeX commands (preamble) between the \documentclass or \documentstyle and the \begin{document} commands.