Turn on Personal Web Sharing from the Sharing Preferences Pane in System Preferences then start Safari and go to:
http://localhost/~username
where username is the your short name i.e. mine is Mark. Next, replace the files already in ~/Sites with your own pages, images etc. You can find a quick intro to Web Sharing here (http://www.apple.com/lae/macosx/whatyoucando/websharing.html).
Thats the basic idea anyway, but if you want to enable PHP and or CGI you'll need to do a little more work :D.
Mark.