You can't. A properly set up server only exposes directories it wants to expose, and you can't even be sure they actually exist; the server might rewrite the URL.
Example: site.com/about/ could be rewritten to site.com/index.php?page=about, so there is no about directory.
You can in theory look for directories by looking at paths in the HTML, for instance if a stylesheet is included like href="css/style.css", it's a pretty safe bet that there's actually a directory called "css" (still, the URL could be rewritten).
Bottom line: there's no way to reliable browse server dirs, unless the admin forgot to close the dirs.
Like if i input yahoo.com, i get all of its file urls.
