Hi there,
Twenty Twelve gives you the option for a “Full-width page, no sidebar” page template. This removes the sidebar from any page you assign that template to. If you want to remove the sidebar from all your posts and pages you just remove any active widgets and the sidebar will disappear.
http://theme.wordpress.com/themes/twentytwelve/
Are you having trouble getting the sidebar to disappear on a particular page or post I can help you with?
Best!