The basic rule
Spanish plurals are built like English ones: you add an ending to the noun, and you can always hear it. The ending is -s or -es, depending on the last letter of the singular. What is new: the article changes too. English the never changes, but masculine el becomes los and feminine la becomes las.
After a vowel: -s
This works for every vowel, including stressed ones: cafés, sofás, menús.
Tengo tres libros nuevos.
I have three new books.
Hay dos coches en la calle.
There are two cars in the street.
Tomamos dos cafés.
We are having two coffees.
After a consonant: -es
Spanish uses -es after every consonant, including d, l, n and r. English only uses it after hissing sounds (bus → buses). With -z the spelling changes too: z becomes c, because Spanish normally writes c rather than z before e. The sound stays the same.
Las ciudades españolas son bonitas.
Spanish cities are beautiful.
Los profesores explican la lección.
The teachers explain the lesson.
Las luces están encendidas.
The lights are on.
The accent in the plural
English writes no accents, so this part is new. The stress itself never moves in the plural, but the written accent can disappear or appear.
Words in -ión lose the accent (canción → canciones): in the plural the stress falls on the second-to-last syllable, which is the default and needs no mark.
joven and examen work the other way round: the extra syllable pushes the stress to the third-to-last position, which always needs a mark (jóvenes, exámenes).
Me gustan estas canciones.
I like these songs.
Los aviones llegan tarde.
The planes are arriving late.
Los jóvenes estudian mucho.
Young people study a lot.
Tenemos dos exámenes hoy.
We have two exams today.
Words that do not change
A word ending in -s that is not stressed on the last syllable does not change at all, like English series or species. Only the article shows the number: el lunes → los lunes.
Los lunes no trabajo.
I don't work on Mondays.
Compro dos paraguas.
I am buying two umbrellas.
