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

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.