Latex: How can I create nested lists which look this 1.1, 1.1.1, 1.1.2, 1.2

No need to use any additional package

\begin{enumerate}
   \item[1.] Topic
   \begin{enumerate}
       \item[1.1] First Subtopic
       \item[1.2] Second Subtopic
    \end{enumerate}
\end{enumerate}

See: http://www.giss.nasa.gov/tools/latex/ltx-222.html

The numbering style for the enumeration is determined by the commands, \labelenumi, \labelenumii, etc., for the nested levels. These may be redefined with the \renewcommand command.

For example, to use upper case letters for the first level and lower case letters for the second level of enumeration:

\renewcommand{\labelenumi}{\Alph{enumi}}
\renewcommand{\labelenumii}{\alph{enumii}}

And here: http://www.mackichan.com/index.html?techtalk/484.htm~mainFrame

... The concrete commands would be

\renewcommand{\labelenumi}{\arabic{enumi}.} 
\renewcommand{\labelenumii}{\arabic{enumi}.\arabic{enumii}}

Or, if you think your content qualifies as sections, use something like:

\section{Name}
...
\subsection{Subtopic}
...
\subsubsection{Yet another nesting}
...

You can use enumitem package:

\documentclass{article}
\usepackage{enumitem}
\begin{document}

\begin{enumerate}
  \item Topic
  \begin{enumerate}[label*=\arabic*.]
    \item First Subtopic
    \item Second Subtopic
    \begin{enumerate}[label*=\arabic*.]
      \item First Sub-Subtopic
      \item Second Sub-Subtopic
    \end{enumerate}
  \end{enumerate}
\end{enumerate}

\end{document}

See the catalog entry for enumitem for more.