it's easy really.. the apostrophe does three things...

firstly it can replace a missing letter

are not can become aren't and the ' replaces the letter o

can not can become can't and the ' replaces the letters n & o

they are can become they're and the ' replaces the letter a

you are can become you're and the ' replaces the letter a

secondly, in front of a letter "S" it can show belonging to a single thing

it was the cat's milk here the ' shows milk belonging to one cat and goes before the S
there were three cats here the apostrophe is not needed (no belonging)

it was the Prime Minister's fault
here the ' shows fault belonging to the PM, there is only one PM, and the apostrophe goes before the S

he crashed Sam's car
here the ' shows the car belonging solely to Sam and goes before the S

thirdly, after the letter "S" it can show belonging to a plural or multiple thing

he blamed the footballers' wives
here the ' shows wives belonging to many footballers and goes after the S

they ate the twins' lunch
here the ' shows lunch belonging to two people and goes after the S

they spent all the students' fees
here the 'shows fees belonging to all the students and goes after the S

as opposed to this: they spent all the student's fees
where the ' show fees belonging to just one student and goes before the S

tricky stuff - for apostrophe superstars only (but by now that's you isn't it?)

firstly - where the word we are adding the apostrophe too is a special word to describe a collection of things. These words don't have an S on the end to make them plurals. Example here is people (one people is a collection of many persons) so here there is only one group - the people - and put the ' before the S

it was the people's will
here the ' shows belonging - with only one people it goes before the S

it was the children's swing
here the ' shows belonging - note there is only one children (so to speak) so it goes before the S

secondly, there are a few exceptions where the belonging rule and the missing letter rule produce the same result. Obviously putting the apostrophe in only one of the identical instances helps readers to tell which one they are reading. You just have to remember which one is right and hope they do too! The example here is with it.

it is a special rule can become it's a special rule.
here the ' shows the missing letter i - this means the ' can't also be used to show possession, so it's OK to write:
he drove the car into its garage
normally there would be an ' to show possession, but it has already been used up to show missing letters (there is a more complex explanation of this, but that one will do for now!). By the way, note that it's OK to write it has as it's too because it and ' always alerts a reader to one or some missing letters, in this case both the h and a.

Gosh, jolly tricky!

thirdly, where the name that is getting the belonging ' ends in an S. Here we don't usually add a second S but we just put the ' after the name.

it was Kris' guitar.
here the ' shows the guitar belonging to Kris, but although the ' is at the end there is only one Kris, not many

fourthly, where the word that is getting the belonging (so to speak) ' ends in double SS. Here we usually do add a third S and we just put the ' after the first two.

Postman Pat brought Jess's milk
here the ' shows the milk belonging to Jess. One Jess only.

Question for black-belt ' aces: where does the ' go if there is more than one Jess?