Here's a solution that typesets the labels in the margin:
\documentclass{article}
\usepackage{enumitem}
\setlist[enumerate]{align=left,leftmargin=0pt,labelsep=0pt,labelwidth=3em}
\setlist[enumerate,1]{label=\alph*.}
\setlist[enumerate,2]{label*=\arabic*.}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{enumerate}
\item First
\item Second
\begin{enumerate}
\item Second-first
\end{enumerate}
\item Third
\end{enumerate}
\end{document}

Addding itemindent=3em
to the example above will make the labels left-aligned at the left margin:
\documentclass{article}
\usepackage{enumitem}
\setlist[enumerate]{align=left,leftmargin=0pt,labelsep=0pt,labelwidth=3em,itemindent=3em}
\setlist[enumerate,1]{label=\alph*.}
\setlist[enumerate,2]{label*=\arabic*.}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{enumerate}
\item First
\item Second
\begin{enumerate}
\item Second-first
\end{enumerate}
\item Third
\end{enumerate}
\end{document}
