magento change form validation messages

1) Add a new class “over_all_fit_required_entry”
<input type=”hidden” name=”over_all_fit_selected” id=”over_all_fit_selected” class=”over_all_fit_required_entry”>

2) Add the below given code
<script type=’text/javascript’>
//< ![CDATA[
Validation.add(‘over_all_fit_required_entry’, ‘ Please select option’, function(v) {
return !Validation.get(‘IsEmpty’).test(v);
});
//]]>
</script>