Your own computer needs to be online for others to access it. Go into sharing preferences, and click on personal websharing. Start it up if it isn't already, and once it is, you'll see a little note just above where you turned it on saying something like 'other users can access your sites from tp://ip-230-246-83-1.yourisp.om (outdated link removed).
You can also view your sites locally by just going to tp://127.0.0.1/~yourusername (I think) (outdated link removed). Any pages you put in your sites directory in your home folder will be shown there. If you want your sites accessible from just 127.0.0.1/ (no username) you can put your html files in Macintosh HD > Library > WebServer > Documents.
If you search on google for apache tutorials you'll find some good stuff. It doesn't matter if you find a tutorial for apache on mac os x or linux, they're basically the same.
If you're still having troubles, I'll post a couple of screenshots for you if you want.