My method is to PRINT "Press A to start" and if the player presses A, it just progress to the new screen with ACLS, FADE, XSCREEN 3, whatever.
Then let the game check if there's a save file (CHKFILE) and if so, PRINT "Press Y to load" and if the player presses Y, use the LOAD command.
You of course need to learn the SAVE/LOAD command and either the TXT or DAT type of save file.
Download my game and see the code for example. (GKNJPRINT is just a customized command for LOCATE and PRINT or GPUTCHR.)
(Line 62)
This line checks if the save file is present(1) or absent(0). If true, it LOADs the file and save it to the array SVFILE. You have to DIM the array first of course.
(Line 242,392,552)
These lines writes a number to the array and save as DAT to be loaded later.
