To number subsubsections and paragraphs, set the secnumdepth counter to 4 (the paragraph level); to get them into the ToC, change the tocdepth counter also to 4; to change the formatting for the paragraph headings, you can use the titlesec package; a little example:
\documentclass{article}
\usepackage{titlesec}
\usepackage{lipsum}
\titleformat{\paragraph}
{\normalfont\normalsize\bfseries}{\theparagraph}{1em}{}
\titlespacing*{\paragraph}
{0pt}{3.25ex plus 1ex minus .2ex}{1.5ex plus .2ex}
\setcounter{secnumdepth}{4}
\setcounter{tocdepth}{4}
\begin{document}
\tableofcontents
\section{Test section}
\subsection{Test subsection}
\subsubsection{Test subsubsection}
\lipsum[2]
\paragraph{Test paragraph}
\lipsum[2]
\end{document}

If, for some reason, using titlesec is not an option, you can redefine \paragraph; here's a little example of this approach:
\documentclass{article}
\usepackage{lipsum}
\makeatletter
\renewcommand\paragraph{\@startsection{paragraph}{4}{\z@}%
{-3.25ex\@plus -1ex \@minus -.2ex}%
{1.5ex plus .2ex}%
{\normalfont\normalsize\bfseries}}
\makeatother
\setcounter{secnumdepth}{4}
\setcounter{tocdepth}{4}
\begin{document}
\tableofcontents
\section{Test section}
\subsection{Test subsection}
\subsubsection{Test subsubsection}
\lipsum[2]
\paragraph{Test paragraph}
\lipsum[2]
\end{document}