Any idea how to get this kind of format?

It looks to me like the screenshot could have been produced with the help of the enumitem package and its \newlist and \setlist macros.

\documentclass{article}
\usepackage{newtxtext,newtxmath} % optional -- Times Roman text and math font

\usepackage{enumitem}
\newlist{spenumerate}{enumerate}{2} % 'spenumerate' -- short for 'special enumerate'
\setlist[spenumerate,1]{label=\bfseries \thesection.\arabic*}
\setlist[spenumerate,2]{label=\bfseries \alph*.}

\begin{document}
\begin{spenumerate}
\item 
  \begin{spenumerate}
    \item a
    \item b
    \item c
    \item d
    \item e
    \item f
  \end{spenumerate}
\item 
  \begin{spenumerate}
    \item A
    \item B
    \item C
    \item D
    \item E
    \item f
  \end{spenumerate}
\end{spenumerate}
\end{document}

The "standard"* configuration method for setting the enumeration label format is redefining \labelenumi (top level), \labelenumii (nested level), etc. Sticking to that basic method gives

\documentclass{article}
% \theenumi is \arabic{enumi}
\renewcommand\labelenumi{\textbf{0.\theenumi}}
% \theenumii is \alph{enumii}
\renewcommand\labelenumii{\textbf{\theenumii.}}
%
\begin{document}

\noindent Hello there. This line shows the left margin.
\begin{enumerate}
\item
  \begin{enumerate}
  \item odd
  \item even
  \item even positive
  \item multiples
  \item pals
  \item empty
  \end{enumerate}
\item
  \begin{enumerate}
  \item alpha
  \item bravo
  \item charlie
  \item delta
  \item echo
  \item foxtrot
  \end{enumerate}
\end{enumerate}
\end{document}

I don't know that your "0" refers to, but you probably want it to give some other counter value.

Note * "Standard" in the sense that it is the style of the built-in document classes. List environments like "enumerate" aren't defined in basic LaTeX., but in the document class.