simply put this above your <form ...> tag:

 <script language="javascript" type="text/javascript">
 <!--
 window.onload = select_me;
 function select_me() {
 	focus();Form.Input.focus()
 }
 //-->
 </script>

 replace 'Form' with the name of your form and replace 'Input' with the name
 of
the input field you want selected.

 that's it!