Validating DataAssist Insert Form

What other event would I do it on? I've tried focus and it doesn't work. Here is my code again what is wrong and what should it be? Thank you for the help. I'm under the gun to get this out today.

<script LANGUAGE="JavaScript">
<!--
function ValidateForm(form){

if ( form.merchant.value == "") { ErrorText= "1";alert ( "Merchant Name is a required field" ); return false; }
if ( form.contact.value == "") { ErrorText= "2";alert ( "Contact Name is a required field" ); return false; }
if ( form.metro.selectedIndex == 0 ) { ErrorText= "3";alert ( "You must choose a Metropolitan Area." ); return false; }
if ( form.terms.checked == false ) { ErrorText= "4";alert ( "Please check the Terms & Conditions box." ); return false; }
}
-->
</script>

<input name="Insert" type="image" id="Insert" onClick="ValidateForm(this.form)" value="Insert" src="images/SubmitButton.jpg" alt="Insert" />