How do you make a word plural in Latin?
First off, you want to have memorized all of the declensions so that on the spot you can say that for example, the plural of ara(altar) is arae or puer(boy) is pueri. Flash cards or online flash cards are good for memorizing endings.
Anyway to the explanation:
Depends on the word. Basically, nouns (and adjectives) come in five declensions
Words of the first declension end in -a in the nominative singular (the form you're likely to come across in non-Latin usages) and -ae in the plural. Example: formula => formulae.
Words of the second declension can end in -us or -er or -um; -us words have -i in the plural (cactus => cacti), -er words have either -ri or -eri (can't think of any English examples; in Latin, you have liber (book) => libri and puer (boy) => pueri), and -um words (gender neutral) have -a in the plural (medium => media).
Words of the third declension are the most complicated; they end in -es or -a in the plural, but the relationship between the singular and plural is not so simple. Examples are homo (man) => homines; corpus (body) => corpora. (Those with the plural in -a are neuter)
Words of the fourth declension end in -us or -u (neuters) in the singular and -us or -ua (neuters) in the plural. Example: apparatus => apparatus (the vowel length changes from short to long).
Finally, words of the fifth declension end in -es in both the singular and plural. Can't think of any that occur in English.
But there are also words of Greek origin that decline according to Greek rules.