If you mean to pick a random letter from a batch of predetermined letters, you can make a one-dimensional array with as many and whichever letters you want, then pick a random element from the array with the RND command.
Example:
DIM LETTERS[0]
PUSH LETTERS,"A"
PUSH LETTERS,"B"
PUSH LETTERS,"C"
PRINT LETTERS[RND(LEN(LETTERS))]
1そうだね プレイ済み