
The full stop's easy - that goes at the end of the sentence, and nowhere else!
The comma has several uses. It's used to separate clauses, where both parts of the sentence are needed to make sense. It's also used where extra detail is added to a sentence and is then used at both ends of the clause, such as: "The dog, which was wearing a new leather collar, ran across the field." The comma is put at each end of the middle clause, but the sentence makes perfectly good sense if the whole clause is lifted out ("The dog ran across the field.").
A comma is also used when you're listing things, but never goes before the 'and' with the last of the list: "The man's breakfast consisted of sausage, bacon, eggs, mushrooms and tomatoes".