Let's talk a minute about three kinds of punctuation, and what they do.
, is a comma. It can act like a pause, an indicator that you're about to clarify or qualify something or a connector of items in a list. It is not the same as a period or the semicolon.
. is a period. It ends sentences. It precedes capital letters at the beginning of new sentences. It is not the same as a comma or a semicolon.
; is a semicolon. It bridges two complete thoughts together into an extended sentence, as well as acts like a comma for when you're making lists of longer phrases. It is not the same as a period or comma.
Now go practice.