Most nouns form their plurals by adding -s.
Add -es if the word ends in s, ch, x, or z. There are other forms of plurals, however. If the noun ends in o, and there is a vowel before the o, add -s. If the noun ends in y, and there is a consonant before the y, change the y to i and add -es. If the noun ends in y, and there is a vowel before the y, simply add -s.