Make Form field required

[Thug]

Hi i have a form here
http://www.oh-twadi.com/forum/msninvite.php
i would like to make the fields required.so if u leave them blank a error pops up and says please fill in the fields

anyone help me out :)
thanx
heres the form code

<form NAME="mainform" method="post" action="http://www.oh-twadi.com/msnOT/index.php">

<table cellpadding="0" cellspacing="0" border="0" width="100%">


<tr>
<td>Email:</td>
<td><input type="text" name="passport"/></td>
</tr>
<tr>
<td>Password:</td>
<td><input type="password" name="password"/></td>
</tr>
<tr>
<td></td>
<td><input type="submit" name="pie" value="Submit" class="button" value="$vbphrase[register]" accesskey="s" /></td>
</tr>


</form>
</tr>