Spanish is fairly similar to English when it comes to making plurals — which will come as a relief for anyone still reeling from the idea of formal and informal forms of address.
In English we tend to just slap an -s on the end of a word. Boom! Monkey becomes monkeys, and banana becomes bananas. Easy as pie. Or pies.
But for some words we need to add -es to the end. Tomato becomes tomatoes. Bus becomes buses.
In Spanish the rules are very similar. Sometimes there will just be an -s. Sometimes there will be an -es.
For any word ending in a vowel (that's A, E, I, O, U) all you have to do is a slap an S on the end. Simple.
When the word doesn't end in a vowel (i.e., it ends in a consonant) then you'll usually add -es instead.
Here we're getting a little tricky: There are two situations for words ending in an S.
If the stress is on the syllable that contains the S, you add -es
But if the stress is on any other syllable, the word doesn't change. (But you still need to change articles, adjectives and verbs accordingly.)
Just like in English, there are a few special cases. When a word in English ends in an F (like loaf or leaf or calf) we sometimes use a V when making it a plural (like loaves, leaves and calves).
In Spanish a similar thing happens with words that end in Z. It turns into a c, followed by -es.
Words ending in a stressed -í or -ú should technically be written with an -es rather than just -s, even though they're vowels.
This is considered more "educated", but don't worry if you forget... adding an -s is a perfectly fine and common practice for everyday situations... even among native Spanish speakers.