\documentclass{article}
\begin{document}
This is my simple document with nested lists. 
\begin{enumerate}
\item First Enumerated Item. 
\item Second Enumerated Item. 
\item Third Enumerated Item. 
  \begin{itemize}
  \item First Item.
  \item Second Item.
  \item Third Item.
    \begin{itemize}
    \item First Sub-Item.
    \item Second Sub-Item.
    \item Third Sub-Item.
    \item Fourth Sub-Item.
    \end{itemize}
\end{enumerate}
\end{document}