To get a list of all sub directories from one location, you can use GetDirectories method, with code like this:
[ C# ]
string[] SiteDirectories = System.IO.Directory.GetDirectories(Server.MapPath("~/"));
[ VB.NET ]
Dim SiteDirectories() As String = _ System.IO.Directory.GetDirectories(Server.MapPath("~/"))
Server.MapPath(“~/”) will simply return root folder of web application. To build more stable application it is recommended to use try-catch block or to check before if directory exists in your specific case.