How to make your LaTeX document look ugly. More specific: The request is to use Arial, 12pt, 1.5 line spacing. Looks very ugly but here it is:

\documentclass[12pt]{scrartcl} % use 12pt
[...]
\usepackage{setspace} % manipulate line spacing
\renewcommand{\rmdefault}{phv} % Arial for serif typeface (normal text)
\renewcommand{\sfdefault}{phv} % Arial for sans-serif (headings)
[...]
\begin{document}
[...]
\onehalfspacing % use 1.5 line spacing
[...]