For the third person singular, use
s.
It's pronunciation will depend on what comes before it: voiced consonant or vowel /z/; unvoiced consonant /s/.
animals, cows; pets
For a word ending in /s/ or /z/ sound, use es (unless the spelling of the word ends in e: then just s. This gives a schwa sound +/z/.
basses [the male voice register]; (phases)
Sounds difficult? Not really. The spelling is simply a reflection of the natural pronunciation.
Welcome to the forums, Mo.