\documentclass{article}
\usepackage[utf8]{inputenc}
\title{Probeersel}
\author{cornelispeek }
\date{July 2018}
\begin{document}
\maketitle
\section{Introduction}
What shall we do \cite{berne} with
\bibliographystyle{plain}
\bibliography{boeken}
\end{document}
You have to name the bibliography file and bibliographystyle before \end{document}
