There are many comma rules, but I will only tell you a few!
1. When separating phrases that don't need to be there.
EX. My friend, Marlen, is really nice.
2. When linking two independent clauses with a conjunction.
EX. I love dogs , but I also love cats!
3. When your addressing someone in particular.
EX. Let's go play, Kacie!
4. When making a list.
EX. I love chocolate, Sweet Tarts, and Kit-Kats!
5. When you have more than one adjective modifying a noun.
EX. The wonderful, beautiful Mrs.Cody is a great teacher!
6. After introductory phrases or clauses.
EX. Once we're done eating, we can go shopping!
7. The date.
EX. July 7, 1998
8. End of a letter.
EX. Come over later, Love Marlen
9. Geographic Locations
EX. Los Angeles, CA
Now, 3 ways NOT to use a comma.
1.When separating 2 independent clauses without a conjunction.
2. After a conjunction.
3.When separating a dependent and independent clause with a conjunction.