Edit the_content function

I want to edit the default function the_content(). I know i can add_filter('the_content', 'with_my_function') but im having trouble outputting the old content with mine.

Heres an example of what im doing.

add_filter('the_content', 'my_function');
function my_function()
{
    $write="hello world";
    $content = apply_filters('the_content', get_the_content());
    echo $write.$content;
}

How can i get this to work?

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.