Most words just need to add an S or ES at the end to form the plural. Here are some of the more unusual plurals in the English language.
Some words that end in Y will change the Y to I and add ES.
Examples: babies, cities, navies (exceptions: keys, donkeys, monkeys)
Some words that end in F will change the F to V and add ES.
Examples: leaves, loaves, selves, hooves (exception: roofs)
Some words that end in a consonant followed by an O will add ES.
Examples: tomatoes, potatoes, heroes, buffaloes, dominoes (exception: pianos)
Latin words have their own rules:
Alumnus, alumni (masculine)
Alumna, alumnae (feminine)
Medium, media (media are plural!)
And then there are some very interesting compound word plurals:
Deputy chiefs of staff
Assistant corporation counsels
The rule of thumb here is to make the most significant word plural.