Generally, s at the beginning of a word is pronounced /s/. it is never pronounced /z/, but is /ʃ/ in sugar, sure.
If it is preceded by a vowel and followed by an unvoiced consonant, it is normally /s/ - paste.
If it is preceded by a vowel and followed by a voiced consonant, it is nomally /z/ - Thursday.
There are so many exceptions to any other rules you may encounter that they are not worth noting.
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