Apostrophes are used for two reasons:
- contractions: Dave isn’t walking his dog. He can’t today— he’s sick.
It’s a shame he is so unwell.
The apostrophe shows that two words have been mashed together, and it is placed in the position where a letter or letters have been taken out.
- ownership: Dave’s dog. Penny’s cat.
If someone’s name ends in s, an apostrophe can simply be added after their name, without adding the extra ‘s’ afterwards: Jules’ car.
The only time an apostrophe should not be used for ownership is when using the pronoun it:
The house was missing its chimney.
Kerry gave the dog its ball.
Apostrophes should never be used to make plurals, or for regular words ending in s.
This means “Dave’s dogs” only needs that one apostrophe after Dave, and “Dave rocks” doesn’t need any.
It can get complicated, though, when one needs to follow multiple conventions at the same time.
It can get complicated when a proper noun needs to be made a plural before the possessive apostrophe is added. For example, the Johnson family live in the Johnsons’ house.
Following the same rule used for Jules’ car, the de Jesus family live in the de Jesus’ house.
If the Weatherby family own a house, it is the Weatherbys’ house.
Here, the family name ends in y, but because it’s a proper noun, the plural is made by simply adding an ‘s’ instead of using the conventional -ies ending for regular nouns that end in y. The apostrophe is then added at the end.
A Quick Guide to Using an Apostrophe
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WordyNerdBird 
There is of course the (apparent) exception of “its”. The possessive form of “it” is “its”, not “it’s”. “It’s” should only be used as a contraction for “it is”. The possessive “its” is not a real exception though, as it’s a distinct word meaning “of it”. He, she, it; his, her, its.