I'm trying to modify the following script...

<form onsubmit="open(this.url.value,'','');return false;">
<select name="url">
<option value ="javascript:void(0)">Please select...</option>
<option value ="http://www.yahoo.com">Yahoo</option>
<option value ="http://www.google.com">Google</option>
<option value ="http://www.apple.com">Apple</option>
</select>
<input type="submit" name="get" value="Go">
</form>

The script is working fine, but I want it to give an alert if they try to submit the form without choosing an option. In other words, select Yahoo and you go to Yahoo. But if you leave it on "Please select..." and press "go you get an alert saying "please select an option".

Thanks for your help!