The old-school method (probably officially deprecated now) is with the FONT tag:
<FONT face="Helvetica,Arial">
<H2>Old School Font Switch</H2>
</FONT>
Just for an alternative view. It's worth bearing in mind that you should list more than one font family, separated with commas; not all systems have all fonts and if you specify a font that the user's browser doesn't have, it'll just use the default. If you have provided a list, the browser will use the first font in the list that it has available. So for a Sans Serif font, you could do:
<P style="font-family: Sans, Helvetica, Arial">foo</P>
Sans takes care of a lot of systems. It's a rare Mac that doesn't have Helvetica, and Windows machines will manage with Arial if they don't have the other two.