How can I create custom an environment with default left margin?

Question

--Edit-- (rewrite the code that I use currently to produce margin left)

This is code that I try to make a new environment (leftwhitespace) of, instead of writing these code that create list with 1 item:

\begin{itemize} 
    \item[]
      line 1
      line 2
\end{itemize}

Want to create new environment leftwhitespace that will do as code above (add margin left):

\begin{leftwhitespace}
    line 1
    line 2
\end{leftwhitespace}

Answer 1

If I understand well what you want, changepage already has an environment for that:

\documentclass{article}
\usepackage{lipsum}
\usepackage{changepage}

\begin{document}

\lipsum[10]
\begin{adjustwidth}{3em}{0pt}
\lipsum[11-12]
\end{adjustwidth}

\lipsum[13]

\end{document} 

Answer 2

Does this solve your query:

\documentclass{article}
\usepackage{lipsum}

\newenvironment{hanglist}[1][\parindent]{%
    \begin{list}{}{%
        \setlength{\leftmargin}{#1}
        \setlength{\labelwidth}{0pt}
        \setlength{\labelsep}{0pt}
        \setlength{\itemindent}{-#1}}
    }{%
        \end{list}
    }

\def\D{\par\noindent\makebox[1em][l]{-- }\hangindent1em}

\newcommand{\detail}[1]{\par\noindent\hangindent=\mylen\hangafter1-- #1}
\newlength{\mylen}
\settowidth{\mylen}{-- }
\begin{document}
\begin{hanglist}[2cm]
\item\lipsum[1]
\item\lipsum[2]
\end{hanglist}

    Regular itemize
    \begin{hanglist}[1cm]
       \item First
       \item Second 
       \item Third
    \end{hanglist}

\subsubsection*{This is a header}
\D This is a detail
\D This is a detail that has too many words in it and consequently runs onto The next line of the page, and I want it to have a hanging indent.

\noindent\textbf{This is a header}
    \detail{This is a detail.}
    \detail{This is a detail that has too many words in it and consequently runs onto the next line of the page, and I want it to have a hanging indent.}
\end{document}