The most reliable way is with a hyperlink, such as this:
<a href="whatever.aspx" target="_new">Click Me</a>
When they click this HTML hyperlink the page will open in a new window.
If that's too complicated for you, you can use this free window opener
control or examine the source code to
see how to open a window with your own code. http://SteveOrr.net/articles/ClientSideSuite.aspx
Be aware that Windows XP SP2 tends to block popup windows so that might
cause you some
hassles in user education.
Also note that many educated users use tabbed browsing these days and prefer
to put their windows where they like. Not that pop-ups can't be appropriate
(namely in application design) but just be aware that you may hit some
usability issues if your have a rather wide audience demographic.