you should write a function that holds the event of onChange for that combobox.
You can get the selected item's value in combobox with
"ocument.form.comboboxname.value"
and the selected text with
"document.form.comboboxname[document.form.comboboxname.selectedIndex].text"

You should make the written function to handle the onChange event.In that funtion you can either write javascript code that works directly on the browser window or make form to be submitted to the web server by form.submit() method.