JavaScript Example   
function validateForm() {   
    var x = document.forms["myForm"]["fname"].value;   
    if (x == null || x == "") {   
        alert("Name must be filled out");   
        return false;   
    }   
}   
   
HTML Form Example   
<form name="myForm" action="demo_form.asp" onsubmit="return validateForm()" method="post">   
Name: <input type="text" name="fname">   
<input type="submit" value="Submit">   
</form>