Problems with the native session

[eluser]Zorancho[/eluser]
Hi there, i have MY_Controller in the libraries folder and in its constructor i am starting a native session like this:
class MY_Controller extends Controller
{
public function MY_Controller()
{
parent::__construct();
session_start();
}
}

It worked well on my server, but then i put the application on real server and it gets me this error:
A PHP Error was encountered

Severity: Warning

Message: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at system/application/libraries/MY_Controller.php:1)

Where i am suppose to start the session before any output starts?
Thanks in advance
Zoran