\documentclass{article}
% the use package commands go after
% the document class
\usepackage{amsmath}
% but before begin document

\begin{document}

\section{Introduction}
\label{sec:Introduction}
Paul was running at $5ms^{-1}$.
$$ 
e^{ \frac{\sum_i i^2}{N} + 2}
$$
Hello world!\\
A new line.   
Not   a    new   line, and I won't 
see             spaces.

\section{Method}
\label{sec:Method}

\section{Results}

In Section \ref{sec:Introduction} we
gave an introduction and in Section
\ref{sec:Method} we outlined our method.

\section{Conclusion}

\end{document}