documentclass[a4paper]{article}
usepackage{geometry}
geometry{a4paper,left=30mm,right=25mm, top=2cm, bottom=2cm}
parindent0mm
usepackage{tabularx}
usepackage{imakeidx}
usepackage{cite}
usepackage{amsmath}
usepackage{hyperref}
usepackage[utf8]{inputenc}
usepackage{lmodern}
usepackage[english]{babel}
usepackage{amsmath}
usepackage{amssymb}
usepackage{bbm}
usepackage{amsthm}
usepackage{color}
usepackage[onehalfspacing]{setspace}
usepackage{graphicx}
usepackage{framed, color}

makeatother
makeindex
begin{document}
section{Test}
cite{test}

bibliography{mainbib}
bibliographystyle{plainnat}
end{document}