Try this:
<HTML>
<HEAD>
<TITLE>Contact Page</TITLE>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
</HEAD>
<script>
function validateForm()
{
var x=document.forms["my_form"]["name"].value;
var y=document.forms["my_form"]["comments"].value;
if (x==null || x=="")
{
alert("name must be filled out");
return false;
}
if (y==null || y=="")
{
alert("comments must be filled out");
return false;
}
}
</script>
<table border="0" cellpadding="0" cellspacing="3">
<form method="post" action="thankyou.php" name="my_form" onsubmit="return validateForm()">
<tr>
<td>Name:</td>
<td><input name="name" type="text"></td>
</tr>
<tr><td>Other Comments:</td>
<td><textarea name="comments" rows=10 cols=30></textarea></td>
</tr>
<tr><td> </td><td><input type="submit" value="Send Message"></td>
</tr></form>
</table>
This will validate on the same page, so it will be best.
EDIT:
For only one, try like this:
<HTML>
<HEAD>
<TITLE>Contact Page</TITLE>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
</HEAD>
<script>
function validateForm()
{
var x=document.forms["my_form"]["name"].value;
var y=document.forms["my_form"]["comments"].value;
var count = 0;
if (!(x==null || x==""))
{
count++;
}
if (!(y==null || y==""))
{
count++;
}
if(count < 1){
alert("Please fill at least one field");
return false;
}
}
</script>
<table border="0" cellpadding="0" cellspacing="3">
<form method="post" action="thankyou.php" name="my_form" onsubmit="return validateForm()">
<tr>
<td>Name:</td>
<td><input name="name" type="text"></td>
</tr>
<tr><td>Other Comments:</td>
<td><textarea name="comments" rows=10 cols=30></textarea></td>
</tr>
<tr><td> </td><td><input type="submit" value="Send Message"></td>
</tr></form>
</table>