If you haven't noticed, Web Sharing has been removed from Mountain Lion. According to Apple, if you need Web Sharing, you should probably look at OS X Server.
Nevertheless, many people want to use Web Sharing, notably to run a local server to test win site design. I've come across two tutorials. One by Brett Terpstra and another by Neil Gee. Finally, Tyler Hall has created a Web Sharing preference pane
Personally, I don't need Web Sharing generally, but I have used it in the past to test web applications. So if you need it, check out the above links.
Mac OS X Hints