please help me when write this code :

$email = new Email_Template();
$email->ss_template = 'ContactUsEmail';
$email->populateTemplate($fields);
$email->subject = 'Subject';
$email->from = 'blah@blah.com';
$email->to = 'blah@blah.com';
$email->send();

then error msg:
[Notice] Undefined variable: fields

please suggest me where declear $fields code, or if wrong code then suggest or send link for help me.........