When to add es or s to make plural?

The great majority of nouns are made plural by simply adding -s to the end of the Word:

• Apple-Apples
• boy-boys
• coin-coins

Words ending in s, z, ch, sh, and x add -es to the end of the Word to form the plural:

• boss-bosses
• waltz-waltzes
• match-matches
• dish-dishes
• box-boxes

There are many other forms of plurals in English that don't use these forms; they're called irregular plurals. See the link below for the variations in forming plurals.