When we want to say that something belongs to a person (or an animal), we normally use 's.
Singular noun + 's
If it is a singular noun or name, we add 's at the end:
my friend's dog my friend is the master of this dog
Kate's blog Kate writes this blog
Mrs Oakley's husband Mrs Oakley is married to this man
cow's milk milk from the cow
Plural noun + '
If it is a plural noun (a person or an animal), we only add an apostrophe ' at the end:
my parents' flat = my father's and my mother's flat
our friends' children = Dan and Rose's children
my sisters' party = the party of my three sisters
If a noun has an irregular plural form (like man–men, child–children), then we add 's.
my children's favourite toys
If one and the same person/thing belongs to more than one person, we add -s only to the last noun, and not to all nouns.
Mike and Lilly's father Mike and Lilly have the same father
my aunt and uncle's house my aunt and uncle live in this house together
If some things belong to some persons, each thing to each person, we put -s after every noun.
Peter's and Bill's desks = Peter's desk and Bill's desk
my cat's and dog's bowls = my cat's bowl and my dog's bowl
Not a person → of
When we speak not about people but about things, we usually use the preposition of. It shows that something belongs to something.
a cup of tea WRONG: tea's cup
the capital of Brazil WRONG: Brazil's capital
the love of my life WRONG: my life's love
the episode of the series WRONG: the series' episode