There are three possible pronunciations for the 3^rd person singular “-s”.

/S/ when the pronunciation of the infinitive ends with a voiceless consonant (without movement of the vocal folds).

/Z/ when the pronunciation of the infinitive ends with a vowel or a voiced consonant (with movement of the vocal folds).

/IZ/ when the pronunciation of the infinitive ends with a sybilant consonant. (/s/, /z/, /ʃ/,  /ʒ/,  /tʃ/, /dʒ/)

You can hear a podcast on the pronunciation of voiced and voiceless consonants.

Practice the pronunciation of the final “-s” on this website:

http://www.better-english.com/easier/thirdpers.htm