Form Validation Code

I'm trying to get this code to work, could sombody please help me. It's supposed to be a validation code for a form.

function validate(){

if (fieldname == ""){

alert("Message ...");

return false;

} else if(fieldname == ""){

alert("Message ...");

return false;

  } else {

return true;

}

}

---- this is ---- the form

<form action="mailto:" method="POST" enctype="text/plain" name="survey" onsubmit="return validate();">

<p>Name* &nbsp; <input type="text" name="visitor" size="40"></p>

<p>Email* &nbsp; <input type="text" name="email" id="email" size="40"></p><br>

<p><textarea name="thoughts" rows="10" cols="40" value="">Feedback</textarea></p>

<p><input type="submit" value="Send Feedback"></p>

</form>
</lj>