To prevent all search bots from indexing a page:
<meta name="robots" content="noindex">
To prevent just Google:
<meta name="googlebot" content="noindex">
If you have control over it, you probably want to add nofollow to links that point to that page as well:
<a href="privatepage.html" rel="nofollow">Link to private page</a>
Theoretically that's not needed for Google, which claims to drop all pages with noindex from their directory. But there are more search engines out there and it doesn't hurt.