When the s is the first letter of a word => the sound is "s" (example : syntaxe)
When the s is the final letter of a word, you have to check for the next first letter, if it's a vowel => the sound is "z" (example : les avions => "les zavions")
When the s is between two letters, then the sound is always "s", except if is it two vowels (example : rose => "roze")
Hope it helps.