How to validate a form

Dear Sir,
I have a one jsp ,in that i have a 2 forms

///One form1 as follows
<form method="post" onsubmit="return IsEmpty(this);">
<center>
<table>
<tr>
<td><b>Id</b><input type="text" name="idvalue" value="<%=sessId %>" ></td>
<td><b>Subject</b><input type="text" name="subject" value="<%=sessSub %>"></td>
<td><b>Status</b><input type="text" name="statusvalue" value="<%=sessStat %>"></td>
<td><input type="submit" value="Search"></td>
</tr>
</table>
</center>
</form>

/// Another form2 is as follows

<form name="mesdetails" method="post" action="../mesdetails.do"><input
type="hidden" name="act" value="" />
<table class="ContentTable" border="4" bordercolor="#660066"
align="center" cellspacing="0" cellpadding="4.5" style="width: 770px"
id="results">

And soon ...

Now i want to validate a form1 i.e in form1 i have a text field for a Id values .If i submit a form without any Id values in a text field i have to get an aleert message so how to do this sir please help me..

Thanks and Regards
Harini Veerapur.