Custom enumeration list Step 1, Step 2 etc

A code with enumitem: I define a steps environment:

\documentclass{article}
\usepackage{enumitem}
\newlist{steps}{enumerate}{1}
\setlist[steps, 1]{label = Step \arabic*:}

\begin{document}

\begin{steps}
  \item first thing to do
  \item Second thing to do
  \item Third thing to do
\end{steps}

\end{document} 

Alternatively, if you don't want to create a new environment, you can use the optional arguments of enumerate:

\begin{enumerate}[label=Step \arabic*:]
\item ...
.........
\end{enumerate}

\documentclass{article}
\usepackage{enumerate}
\begin{document}
\begin{enumerate}[Step 1:]
\item First thing to do
\item Second thing to do
\item Third thing to do
\end{enumerate}
\end{document}