This tool makes it easy to create a “Share on Facebook” button or link. It uses Facebook's old “sharer.php” method, which is now undocumented and probably deprecated. But it works.
The alternative: The “proper way to do it” nowadays is the
Feed Dialog Share dialog. The Share Dialog method has lots of cool features and options, and it is also possible to link to it via direct URL. However, it's a bit more complicated, you'll have to provide an application ID and users can't choose where to post to.