If you mean pronunciation of a letter 's' at the end of words, it is more or less as follows:
1) 's' is pronounced as /s/ when follows voiceless consonants; e.g. cats ('t' is a voiceless consonant) --> /kæts/
2) 's' is pronounced as /z/ when follows voiced consonants; e.g. dogs ('d' is a voiced consonant) --> /dɒgz/
3) 's' is pronounced as /ɪz/ when follows 'hissing' sounds /s,z,ʃ,ʒ,ʧ,ʤ/; e.g. juices, causes, abolishes, camouflages, watches, bridges --> /'ʤu:siz/, /'kɔ:ziz/, /ə'bɒliʃiz/, /'kæm.ə.fla:ʒiz/, /'wɒʧiz/, /briʤiz/
I hope it helps.