[writing a plugin] place a text after posts before comments

Hi! I’m learning writing plugins: if I want to add “hello” every posts at the end of content before comments: which code I need:

I tried:add_filter('the_content','my_plugin_function');

but it replace the content of post with ‘hello’, which action or filter I have to add to place it after each post?? thanks if you help me

plugin_activate()
function plugin_activate()
{

	add_filter('the_content','my_plugin_function');

}

function my_plugin_function() {

	$out = 'hello';
	$out = apply_filters('write_content', $out, $arg);
        return $out;
}