Get post content outside the loop

i want to get the post content via AJAX when the user click a post.
in my theme folder i created a getpost.php:

<?php
	$id = (int)$_GET['post_id'];
	$post = get_post($id);
	echo $post->post_content;
?>

Then i call it like:

$.ajax({
	url:themepath+'/getpost.php?post_id='+id,
	success:function(data){
		alert(data)
		}
	});

i also tryied query_posts('p='.$id);, get_post_field( 'content', $id );

but they all return a error like Fatal error: Call to undefined function get_post()