Edit the_content function

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.