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.