Apostrophes can be used in two ways, to show possession or to make a contraction.
Possession
Basic Possession
Bob's shop
Rose's address
Cleo's dogs
Plural Words ending in "s"
Rock fans' t-shirts (multiple rock fans)
The bakers' convention (multiple bakers)
The students' injuries (multiple students)
Singular words ending in "s."
If a word ends in "s" and is singular, you have two options: add an 's or only add an apostrophe. Choose whichever sounds better.
The James' house on Elm Street. (The double "s" sounds shaky.)
Ross's new car is sweet. (The double "s" sounds okay.)
Gus's room has a weird smell. (double "s" sounds okay.)
Contractions
A contraction is used when two words become one.
Can't = can not
It's = it is
Could've = could have