I'd start by adding an empty List to the layout with a List OnTouch defined (more on that later).
Invoke a function (perhaps named F) passing it the path of your start folder
Function F will receive a path and do a ListFolder putting the files and subfolders in the List.
In the list OnTouch, if the user touches a file then do whatever your app needs to do.
If the user touches a subfolder then invoke F passing it the path of the subfolder).
If the user needs to go back then you need a button (disabled initially and when the user returns to the top, enabled for subfolders)
When the user touches the back button the button OnTouch should invoke F passing it the folder one level up.
Regards, ah