As you might discover from searching the boards, the "headers already sent error" means that you have something being outputted before the command you're trying to call (in this case, session_start().
As the error message states, the output which caused headers to be sent was on line 1 of index.php. Eliminate or move the output from line 1 (be it an HTML tag, empty space/line, Unicode BOM marker, etc.) and that error message should go away.