Spanish nouns follow some patterns when they become plural. However, there are exceptions, especially when it comes to foreign words.
GENERAL RULES AND THEIR EXCEPTIONS
Spanish nouns become plural by adding –s or –es to the singular form.
1. Nouns that end in an unstressed vowel or a stressed –a, –e, or –o become plural by adding –s.
mesa → mesas
sofá → sofás
pozo → pozos
2. Nouns that end in a stressed –i or –u become plural by adding –es.
The plural form with only –s is acceptable, but adding –es is recommended, especially in formal speech.
esquí → esquíes (or esquíes)
tabú → tabúes (or tabús)
Some foreign or colloquial words that end in a stressed –i or –u become plural by adding only –s.
pirulí → pirulís
champú → champús
3. Nouns that end in a consonant generally become plural by adding –es.
reloj → relojes
rey → reyes
*At the end of any word, –y is pronounced as a vowel, yet still follows this rule and adds –es to become plural. Then, when followed by –es, the –y is pronounced as a consonant.
Remember, if the singular form ends in –z, then change –z to -c before adding –es.
avestruz → avestruces
lápiz → lápices
Foreign words that end in a consonant often become plural by adding only –s, especially if the consonant is not a common ending consonant in Spanish* or is preceded by another consonant.
*The most common ending consonants in Spanish are -d, -n, -l, -r, -s and -z.
chat → chats
iceberg → icebergs
When the ending letter is –y preceded by a vowel, it becomes –i.
jersey → jerséis
espray → espráis
For nouns that end in –s preceded by an unstressed vowel, the singular and plural form are the same.
atlas → atlas
oasis → oasis
This group includes the names of the days of the week that end in –s:
lunes, martes, miércoles, jueves, viernes
It also includes compound nouns formed by a noun already in its plural form:
espantapájaros, paraguas, lavaplatos
ACCENTUATION OF PLURAL NOUNS
The stressed vowel in the plural form of a noun is the same one as in the singular form.
(stressed syllables are underlined)
sofá → sofás
canal → canales
cama → camas
lápiz → lápices
pájaro → pájaros
sílaba → sílabas
There are three exceptions:
carácter → caracteres
espécimen → especímenes
régimen → regímenes
In some cases, to preserve the position of the stress, and due to the change in the number of syllables when adding -es, the plural form needs a written accent that was unnecessary in the singular form, or else loses the written accent that was necessary in the singular form:
imagen → imágenes
camión → camiones
I think these are the most important points for how to make Spanish nouns and adjectives plural. Though anybody could naturally learn how to make plural nouns and adjectives by reading and listening, I recommend that all beginner students learn these rules and exceptions.