Hi. You'll need to use JavaScript to validate your form if you want to validate before the submission takes place.
For example:
<SCRIPT type="text/javascript">
function submitForm()
{
if(document.getElementById('FIRST_NAME').value=="")
{alert("Sorry, you need to type in your name.");}
else{document.FORM1.submit();}
}
</SCRIPT>
<FORM NAME="FORM1">
Enter your name: <INPUT TYPE="TEXT" NAME="FIRST_NAME" ID="FIRST_NAME">
<BR>
<INPUT TYPE="BUTTON" onClick="submitForm();" VALUE="SUBMIT">
</FORM>
If you click on the SUBMIT button, it pops up an error if you havn't entered in your name, otherwise it submits.
This example is pretty simple, but hopefully it gets you on the right track.

Cheers.