You need to use the starred version * of the figure environment:
\documentclass[twocolumn]{article}
\usepackage[showframe]{geometry}% http://ctan.org/pkg/geometry
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\begin{document}
\lipsum[1-2]
\begin{figure*}
\includegraphics[width=\textwidth,height=4cm]{tiger}
\caption{This is a tiger.}
\end{figure*}
\lipsum[3-10]
\end{document}

This will usually flush the figure to the top of the following page, so there's not much control left to the user for movement. However, this may just be dependent on the user output.
In my opinion, you may be better off (for ease of use) with the multicol package. Then you can specify text (and sectional commands) in a multicols environment (with a mandatory argument specifying the number of columns), while specifying your figures in the usual way. The layout is different, since the content flows now with the multicols environment. However, this may also be user preference.
Here's a minimal working example:
\documentclass{article}
\usepackage[showframe]{geometry}% http://ctan.org/pkg/geometry
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{multicol}% http://ctan.org/pkg/multicols
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\begin{document}
\begin{multicols}{2}
\lipsum[1-2]
\end{multicols}
\begin{figure*}[h]
\includegraphics[width=\textwidth,height=4cm]{tiger}
\caption{This is a tiger.}
\end{figure*}
\begin{multicols}{2}
\lipsum[3-4]
\end{multicols}
\end{document}

\usepackage{graphicx}you can\begin{figure}[!ht] \centering \includegraphics{figures.png} \caption{Caption!} \label{fig:test} \end{figure}
– bim Jan 24 '21 at 14:13