You can follow this links for a formal introduction to javascript:

http://www.tizag.com/javascriptT/

To include a javascript file inside <header></header)

To change a div element in a form

• form1 = document.getElementById("FORM1");
  document.getElementById('DynamicHiddenField').innerHTML =
  "<input type='hidden' name='delete_file_id' value='"+ fileid +"'>";
  
  form1.submit(); /* finally resubmit the form */

JAVA SCRIPT reference

http://www.w3.org/TR/REC-html40/interact/scripts.html