In most cases we make a plural noun by adding s to a singular noun (car > cars).
Words that end in -ch, x, s or s-like sounds take -es for the plural (kiss > kisses).
When a noun ends in y we replace it with –ies to make the plural (city > cities).
There are also a lot of irregular nouns which do not follow the above rules, so it's a case of you having to learn them one by one!
There are some nouns that form the plurals differently.