autoloading or loading session class and url helper

[eluser]midooh[/eluser]
I"m new to CodeIgniter everytime I autoload or load session class and url helper They doesnt work I get the message

A PHP Error was encountered
Severity: Warning

Message: Cannot modify header information - headers already sent by

Filename: libraries/Session.php

Line Number: 662

and

A PHP Error was encountered
Severity: Warning

Message: Cannot modify header information - headers already sent by

Filename: helpers/url_helper.php

Line Number: 539


Every time I try the redirect example it doesnt work
can u help me with it plz