Answer: Use a period at the end of a sentence. You typically use periods when your declaring something.
Examples: I like pizza. I am going to Hawaii next summer.
Use a comma in several different circumstances such as before a coordinating conjunction (FANBOYS), adding on to a sentence, and typically where you want the reader to pause and take a breath.
Example: My favorite sports are football, basketball, and volleyball.
When it is raining you should use an umbrella, or you will get wet.