Not Java, JavaScript. They're totally different - the only thing in common is the name. In the <a> tag, simply put:
onClick="javascript:window.open('<url>'); return false;" As you can probably see, this opens the window you place in the brackets. The "return false" bit tells the link not to go wherever it was going - you still put the href bit in for people who don't use JavaScript.
An alternative (and probably a better idea) is to put this into a bog-standard link:
With the JavaScript, you can change the width and the height of the window, but someone else is gonna have to help you with that.