\documentclass[aip, % Title and author aligned to the left
amsmath,amssymb, % Maths stuff
reprint, % 2 column and title not on its own page
author-year % for the reference list
]{revtex4-1}

\usepackage{graphicx}
\usepackage[paperwidth=210mm,paperheight=297mm,centering,left=1.8cm,top=1.8cm,right=1.8cm,bottom=1.8cm]{geometry} % a4 paper and margins

%%% If you want it to all use sans font
%\usepackage{sansmathfonts}
%\usepackage[scaled=0.95]{helvet}
%\renewcommand{\familydefault}{\sfdefault}

\linespread{1.2} % Spacing

% Changes table and figure names and numbering
\renewcommand{\figurename}{Figure}
\renewcommand{\tablename}{Table}
\renewcommand{\thetable}{\arabic{table}}
\renewcommand{\thefigure}{\arabic{figure}}

\usepackage{lipsum} %%%% Delete this thing

\begin{document}

\title[Header title
]{Title}

\author{Author}

\date{\today}

\maketitle

\section{Section 1}

\lipsum

\section{Section 2}

\begin{figure*}
\includegraphics{figure1.png}
\caption{caption for figure 1 - This is a wide figure using both columns}
\end{figure*}

\lipsum[1]

\begin{figure}
\includegraphics{figure2.png}
\caption{caption for figure 2 - This is a figure in just 1 column}
\end{figure}

\lipsum[2]

\begin{table}
\begin{tabular}{|l|l|l|l|}\hline
This is & Just a & Random & Table \\ \hline
a & b & c & d \\
1 & 2 & 3 & 4 \\ \hline
\end{tabular}
\caption{caption for table 1 - single column}
\end{table}

\lipsum[3-6]

\begin{table*}
\begin{tabular}{|l|l|l|l|}\hline
This is & Just a & Random & Table \\ \hline
a & b & c & d \\
1 & 2 & 3 & 4 \\ \hline
\end{tabular}
\caption{caption for table 2 - double column}
\end{table*}

\section*{Section without a number}

\lipsum[7-10]

\end{document}