The target
attribute is your best way of doing this.
<a href="http://www.starfall.com" target="_blank">
will open it in a new tab or window. As for which, it depends on the users settings.
<a href="http://www.starfall.com" target="_self">
is default. It makes the page open in the same tab (or iframe, if that's what you're dealing with).
The next two are only good if you're dealing with an iframe.
<a href="http://www.starfall.com" target="_parent">
will open the link in the iframe that the iframe that had the link was in.
<a href="http://www.starfall.com" target="_top">
will open the link in the tab, no matter how many iframes it has to go through.