Quick and simple. Add an 's to any word that owns something. For example:
"That book belongs to John" can be rewritten as "That book is John's."
Note the 's on the end of John.
Now, the tricky part really isn't that tricky. If the word already ends in 's', just add an apostrophe.
"Thomas' bike is red."
Is there an exception? Of course there's an exception! This is the English language we're talking about... :D The exception is if the word is one syllable (kiss versus kiss-es; or cat versus kit-ten).
So, here are more examples, both the ones following the rule and the exceptions.
My boss's job.
The class's pet.
The cactus' spines.
The goddess' love.
A mistress' dress.