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))]
It's better to use methods that take up less memory. Though Oscar's method does work and has some advantages, here's my recommendation: use CHR$ combined with RND.
PRINT CHR$(RND(26)+65)
This can be slightly adjusted to use lowercase letters:
PRINT CHR$(RND(26)+97)
Then you'll use less memory.