At the point in your program at which you want the Text file to open, for example:
(defun c:test ( )
(getstring "\nPress Enter to open Notepad...")
(startapp "notepad")
(princ)
)
Or, as a better example:
(defun c:test ( / file )
(if (setq file (getfiled "Select Text File to Open" "" "txt" 16))
(startapp "notepad" file)
)
(princ)
)