I've got a blog on something.wordpress.com , I merely have to have one publish to continually remain on the top. Other posts should use your regular order. Can someone suggest how to achieve that Or perhaps a theme making this possible/?

Sticky posts really are a builtin feature of WordPress. Edit the visibility of the publish, and you will see.

I am unsure this is actually a programming question, however it appears like you will get several plug ins for Wordpress that will help you to set a "sticky" publish. Start here: http://wordpress.org/extend/plugins/search.php?q=sticky