I would add a rule under CONTENTS
like this:
How can I do? I have to use some particular package?
The following code may suit your needs:
\documentclass{article}
\usepackage{tocloft}
\renewcommand\contentsname{Contents}
\renewcommand\cfttoctitlefont{\Large\bfseries}
\renewcommand\cftaftertoctitle{\medskip\hrule\medskip}
\begin{document}
\tableofcontents
\section{Test}
\end{document}