This way:
\documentclass[12pt,twocolumn]{article}
\usepackage{float}
\usepackage{lipsum} % provides just dummy text
\begin{document}
\twocolumn[{\begin{figure}[H]
\setlength{\linewidth}{\textwidth}
\setlength{\hsize}{\textwidth}
\centering
\rule{10cm}{5cm} % this is your image
\caption{My first float}
\end{figure}}]
\lipsum % provides just dummy text
\end{document}
How does it work:
The command \twocolumn initializes twocolumn mode. It has an optional argument that gets typeset as a "header" in the onecolumn mode.
In this header we put a float placed Here (please, never use H specifier elsewhere).
Inside the float, we have to convince LaTeX that it's a wide figure, by correctly redefining the two width parameters.
Version with \maketitle
The difference is that instead of using the command \twocolumn, we add the figure into the command \@maketitle which is responsible for typesetting the title material.
\documentclass[12pt,twocolumn]{article}
\usepackage{float}
\usepackage{lipsum} % provides just dummy text
\title{MyTitle}
\author{Me the 1st}
\begin{document}
\makeatletter
\g@addto@macro\@maketitle{
\begin{figure}[H]
\setlength{\linewidth}{\textwidth}
\setlength{\hsize}{\textwidth}
\centering
\rule{10cm}{5cm} % this is your image
\caption{My first float}
\end{figure}
}
\makeatother
\maketitle
\lipsum % provides just dummy text
\end{document}