You basically write the data from your player to a file. Lets say you want to save the player's name, that would a a string (a string is something that's made from letters, and enclosed between "inverted commas", i.e. "ajrm007" ), you might also want to write the player's positions in the level, that would be a float (a float it a number that can have decimals, like 3.1415926537). You may lastly want to save which level your player is in, that would be an integer (a number that cant have any decimals, i.e. 144, you would say byte when you write this).
So to save:
open to write "save.txt",1
write string 1,name$
write float 1,posx#
write float 1,posy#
write float 1,posz#
write byte 1,currentLevel
close file 1
And to load:
open to read "save.txt",1
read string 1,name$
read float 1,posx#
etc.
close file 1