Here's a form validation script courtesy of The JavaScript Source
You can find the script here:

http://javascript.internet.com/forms/basic-validation.html

 

 

1. Paste this part between your <HEAD> </HEAD>tags.
A good place might be right after the title <TITLE></TITLE> tags

 <SCRIPT LANGUAGE="JavaScript">
<!-- Original: wsabstract.com -->
<!-- This script and many more are available free online at -->
<!-- The JavaScript Source!! http://javascript.internet.com -->
<!-- Begin
function checkrequired(which) {
var pass=true;
if (document.images) {
for (i=0;i<which.length;i++) {
var tempobj=which.elements[i];
if (tempobj.name.substring(0,8)=="required") {
if (((tempobj.type=="text"||tempobj.type=="textarea")&&
tempobj.value=='')||(tempobj.type.toString().charAt(0)=="s"&&
tempobj.selectedIndex==0)) {
pass=false;
break;
}
}
}
}
if (!pass) {
shortFieldName=tempobj.name.substring(8,30).toUpperCase();
alert("Please make sure the "+shortFieldName+" field was properly completed.");
return false;
}
else
return true;
}
// End -->
</SCRIPT>

2. Add the following to the form tag <FORM>:

onSubmit="return checkrequired(this)"

The completed form tag should look like this:

<FORM METHOD="POST" ACTION="/bin/script_library/form_handler_mail"
onSubmit="return checkrequired(this)">

3. For every field you wish to require attach the word required to the field input name like this:

INPUT NAME="requiredEMAIL" TYPE="text" ......

 

It's a good idea to mark the required fields in some way to guide visitors filling out the form (ie different color text, an *, or the word "required")