If there's just one post in particular you want to have at the top, there's a WordPress plugin that can do it for you:
WP-Sticky. It's for WP 2.5 or higher, but if you haven't upgraded and don't want to, earlier versions are still available (see 'versions' link in the sidebar on the plugin's page at wordpress.org).
And if you're not on WordPress, some platforms let you add text widgets to the top of the page. If all else fails, you can edit your template by hand (being sure to take a backup copy first!) to add an 'announcement' to the top.