The orthographic reform of the German language in 1990 simplified the use of ß (the spicy s, or eszett), and introduced more logical rules for language learners.
Use ß when…The sound /s/ is written ß when it follows a long vowel or diphthong:
weiß (white) ; Fuß (foot) ; fließen (to flow) ; schießen (to shoot) ; büßen (to pay)
ß can never be at the beginning of a word.
Use ss when…The sound /s/ ss is written after a short vowel:
dass (that) ; Fass (barrel) ; messen (to measure) ; Riss (tear)
In Swiss German, the question does not arise- ß does not exist.