The pronunciation of the Spanish letter S is pretty straightforward. As in English, there are two pronunciations.
1. The S is usually "soft," like the English S in words like house and some. IPA [s]
2. When the S precedes a voiced consonant (b, d, g, l, m, n), it is a "hard" sound, like the S in rose and cause. IPA [z]
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.