This example creates a folder and a subfolder on a computer.

        // Specify a "currently active folder"
        string activeDir = @"c:\testdir2";

        //Create a new subfolder under the current active folder
        string filePath = System.IO.Path.Combine(activeDir, "mySubDir");

        // Create the subfolder
        if (!System.IO.Directory.Exists(filePath)) {
                  //Create directory
                  System.IO.Directory.CreateDirectory(filePath);
        }

For more info see: http://msdn.microsoft.com/en-us/library/as2f1fez.aspx