Try by giving id attribute to the elements, make sure id attribute values are unique. Use document.getElementById('idValue') and use .checked for check boxes and redio buttons, use .selectedIndex to get the selected index for list boxes.
Try looking at the material on HTML DOM [Dynamic Object Model] available at the following URL
http://www.w3schools.com/htmldom/default.asp
But i would suggest to avoid java script validation and use server side validation.
- Rakesh