How to refer to sections

Use \label to add label to a section and \ref to refer to it.

\section{Greetings}
\label{sec:greetings}
 
Hello!
 
\section{Referencing}
 
I greeted in section~\ref{sec:greetings}.

 see http://en.wikibooks.org/wiki/LaTeX/Labels_and_Cross-referencing for more detils