<form method="POST" id="form1" action="target.php">
...
<input type="submit" name="submit" value="Submit"/>
</form>

function validateForm()
{
    var x=document.forms["myForm"]["fname"].value;
    if (x==null || x=="")
    {
       alert("First name must be filled out");
       return false;
    }
}

<form name="myForm" action="demo_form.asp" onsubmit="return validateForm()" method="post">
First name: <input type="text" name="fname">
<input type="submit" value="Submit">
</form>