Have a look through the powerpoint that talks about commas. The main thing I want you to focus on is avoiding using a comma splice. A comma splice is a comma used incorrectly when it separates two main clauses.
A main clause is a sentence that makes sense on its own. If it makes sense on its own it should be its own sentence separated with a full stop and not a comma.
The boys went to go to the shops, they then went to play football.
Both of those make sense on their own so they should be their own sentence.