How To Use Commas

Commas are used to separate parts of a sentence.

Lists

I like apples, pears, bananas and oranges. 

Joining clauses

After dinner, we went to the theatre.
(subordinate clause), (main clause)

I heard about them in Manchester, where I used to live.
(main clause), (subordinate clause)

To add more detail inside a sentence

My Dog, Rover, is very fat.

Adding a pause

Mr Smith, please come over here.

When not to use a comma

Commas are not used with essential relative clauses. In other words, If the sentences is not complete without it.

That's the girl who told me.