From the comments, you are using report
(by the way, the class option is oneside
and not onesided
), so your higher sectional unit (assuming you won't be using \part
) should be produced with \chapter
; lower sectional units can be produced using \section
, \subsection
, \subsubsection
, paragraph and \subparagraph
(each one of these has its counter subordinated to the counter of the unit immediately above):
\documentclass[12pt,oneside]{report}
\setcounter{tocdepth}{5}
\setcounter{secnumdepth}{5}
\begin{document}
\chapter{Introduction}
\section{A test section}
\subsection{A test subsection}
\subsubsection{A test subsubsection}
\paragraph{A test paragraph}
\subparagraph{A test subparagraph}
\subsection{Another test subsection}
\section{Another test section}
\chapter{Another test chapter}
\section{A test section}
\section{Another test section}
\subsection{A test subsection}
\subsection{Another test subsection}
\end{document}

Perhaps you want to change the way chapter titles are formatted by suppressing the word "chapter" and placing the title in the same line with the number? If this is so, the titlesec
package can do the job for you:
\documentclass[12pt,oneside]{report}
\usepackage{titlesec}
\titleformat{\chapter}[block]
{\normalfont\huge\bfseries}{\thechapter}{20pt}{}
\titlespacing*{\chapter}
{0pt}{20pt}{20pt}
\setcounter{tocdepth}{5}
\setcounter{secnumdepth}{5}
\begin{document}
\chapter{Introduction}
\section{A test section}
\subsection{A test subsection}
\subsubsection{A test subsubsection}
\paragraph{A test paragraph}
\subparagraph{A test subparagraph}
\subsection{Another test subsection}
\section{Another test section}
\chapter{Another test chapter}
\section{A test section}
\section{Another test section}
\subsection{A test subsection}
\subsection{Another test subsection}
\end{document}

Another option, would be to use article
instead of report
and \section
, \subsection
, \subsubsection
, \paragraph
and \subparagraph
(there's no \chapter
in article).
chapter
likereport
orbook
. If you want to avoid this level you can use a document class based onarticle
.chngcntr
package. this question gives good information on how to use it.