How to display section number in the section's title?

You mean something like this? Using titlesec will make automatically every section use the "Item xx" value.

\documentclass[a4paper]{article}
\usepackage{titlesec}
\titleformat{\section}[hang]{\bfseries}{Item \thesection:\ }{0pt}{}

\begin{document}

\section*{Introduction}

    Introduction text here...

\section{What is Quantitative Finance?}
Indeed, before actually studying a topic, we should start by having a good definition of what we are studying. So what is quantitative finance?

\section{Geometric Brownian Motion}
blalblabla

\end{document}

No package usage here, just slightly redefining \@seccntformat and checking with \pdfstrcmp whether it is section or another level.

\pdfstrcmp is a primitive provided by pdftex and pdflatex, so it is a built-in command.

\documentclass[a4paper]{article}


\makeatletter
\let\@seccntformatorig\@seccntformat
\def\@seccntformat#1{%
  \ifnum0=\pdfstrcmp{#1}{section}%
  Item \csname the#1\endcsname:{} %Well, could say Item \thesection:{} here as well... 
  \else
  \@seccntformatorig{#1}%
  \fi
}
\makeatother

\begin{document}




\section*{Introduction}

    Introduction text here...

\section{What is Quantitative Finance?}
Indeed, before actually studying a topic, we should start by having a good definition of what we are studying. So what is quantitative finance?

\subsection{Foo}


\section{Geometric Brownian Motion}
blalblabla

\end{document}

No packages and the possibility of doing different things for different levels. Here I define \formatsection to add Item n: and \formatsubsection to do Subitem n.m:, whereas the standard output is used for \subsubsection.

\documentclass[a4paper]{article}

\makeatletter
\renewcommand{\@seccntformat}[1]{%
  \ifcsname format#1\endcsname
    \csname format#1\endcsname
  \else
    \csname the#1\endcsname\quad
  \fi
}
\makeatother

\newcommand{\formatsection}{Item \thesection: }
\newcommand{\formatsubsection}{Subitem \thesubsection: } % just for example

\begin{document}

\section*{Introduction}

Introduction text here...

\section{What is Quantitative Finance?}

Indeed, before actually studying a topic, we should start by 
having a good definition of what we are studying. So what is 
quantitative finance?

\subsection{This is a subsection}

Here we have text.

\subsubsection{This is a subsubsection}

Here we have text.

\section{Geometric Brownian Motion}

blalblabla

\end{document}