With Leopard, the System Preferences pane is different.  There is now just a single option for web sharing.  Open System Preferences, Sharing and enable Web Sharing.  Apache2 is installed as a default.  You should see your local website using this in the browser (using your own user short name in place of “shortname”.

localhost/~shortname

If you upgrade to Leopard from a previous version of OS X, then you may get a “Forbidden 403″ message” instead of seeing your web page.  To fix go to this solution on the Apple website.

http://support.apple.com/kb/TA25038?viewlocale=en_US

Your web pages are stored in the folder /user/shortname/sites.  A default html page is located there, which should now come up.