In Spanish, you will never see an apostrophe "s." Possession is shown in a couple of different ways. One way is to use the preposition de (of) when you are expressing the possession in third person.
Example: el libro de Paco. = Paco's book
Another way to show possession is by using "possessive adjectives." Possessive adjectives are used to either establish a relationship between people or things, or to indicate that something belongs to someone. Read the sentences below and note how the possessive adjectives (mi and mis) are used.
Éste es mi gato. = This is my cat.
Éstás son mis primas. = These are my cousins.
When deciding which possessive adjective is needed, select the one that indicates the owner. Then, add an the letter -s- at the end if what is OWNED is plural. The possessive adjectives nuestro (our) and vuestro (your -vosotros form) also reflect the gender of what is owned. For example:
our house = nuestra casa / our car = nuestro auto
our houses = nuestras casas / our cars = nuestros autos