Here's the laymans' guide to using possessive apostrophes.
When we want to talk about something owned or posessed by someone we use an apostrophe before the S.
Example: If we want to talk about a balloon owned by a boy we say:
The boy's balloon
If we want to talk about something owned by more than one person we put the apostrophe after the S
Example: If we want to talk about a football owned by two brothers we might say:
The boys' football
When names end with the letter 's', you can put the apostrophe in one of two places.
James' fish tankor James's fish tank
The apostrophe is never used with possessive pronouns like: his, hers, its, ours, yours or theirs:
You would never write Hi's ball