From: yourdictionary.com "Apostrophe Rules - English Grammar Rules & Usage"
My edits indicated with [brackets].
Use an apostrophe + "s" ('s) to show that one person/thing owns or is a member of something.
•Amy's ballet class
•Ross's sports teams
Even if the name ends in "s," it's still correct to add another "'s" to create the possessive form. It is also acceptable to add only an apostrophe to the end of singular nouns that end in "s" to make them possessive. In this case, you can show possession for Ross either way:
It makes no difference whether the item owned is singular or plural. We use "Ross's" to say that the room (singular) is his and that the sports teams (plural) are his.
Use an apostrophe after the "s" at the end of a plural noun to show possession.
•The parents' bedroom
•The Smiths' lives
It is not necessary to add another "s" to the end of a possessive plural noun.
If a plural noun does not end in "s," add an apostrophe + "s" to create the possessive form.
•The children's rooms
Remember, a possessive noun [should have] an apostrophe and an "s" at the end. [Alternatively,] if there's already an "s" there, you can just add the apostrophe. If there's no "s," you have to add both - first the apostrophe, and then the "s."