In preamble, I have:
\renewcommand{\familydefault}{\sfdefault}
\usepackage{arial}
but it doesn't work.
How do I set my font to Arial throughout the entire document?
UPDATE:
In short, both answers work.
It really depends on the environment. If using LaTeX and don't mind using a cloned font then choose Herbert's answer. If using XeLaTex and want to use the authentic font then choose Leo's.