From time to time I end up getting confused as to when one should use the apostrophe, so here is a nice post which explains it all. Basically the main difference is between possessive pronouns and singular and plural nouns.

To show possession for singular nouns (ending in s or not):

– that person’s leg was injured
– my dog’s tail was wagging
– my boss’s house is amazing
**
To show possession for plural nouns (ending in an s):**

– the girls’ swing
– the students’ project

To show possession for plural nouns (not ending in an s):

– the children’s toys

To show possession for names (ending in s or not)

– the Hastingses’ appointment
– the Browns’ fish
– uncle John’s boat
**
To show possession for possessive pronouns:**

– yours
– his
– hers
– its*
– It’s the first day of spring. (it is)
– Our bird has escaped from its cage. (possession for its)
– ours
– theirs

exceptions for some indefinite pronouns

– anybody’s guess
– one’s personal responsibility
– somebody’s wallet

Hope this helps
Cheers