Not exactly sure what you are trying to accomplish, but it looks like you are trying to prepend something to the beginning of the_content.
Try this:
add_filter('the_content', 'se24265_my_function');
function se24265_my_function( $content )
{
$write="hello world";
$new_content = $write . $content;
return $new_content;
}
Most filters will provide a parameter or two for your function to manipulate and then return.