Blogs are essentially a reverse chronological order publishing tool and you cannot change this order on a free hosted WordPress.com blog. Posts will be posted to your blog in accord with their datestamps at publication. WordPress by default shows posts in reverse chronological order (newest post on top).
If your posts are not displaying in reverse chronological order then check your date and timezone and reset if required on this page > Settings > General
If that's not an issue then you could have a sticky posts issue. Simply remove the sticky designation. http://en.support.wordpress.com/posts/post-visibility/#sticky-posts