It is conventional to use commas in the following contexts.

In place names and addresses:

The mailing address for FBI headquarters is 935 Pennsylvania Avenue NW, Washington, DC, 20535-0001.

In dates:

On September 10, 1995, newspapers published the Unabomber’s manifesto.

In numbers (except years, zip codes, and street numbers) of four or more digits:

DNA testing is conducted on only small sections of the human genome, which is about 3,200,000,000 base pairs long and contains 20,000–25,000 genes.

Between two words that might otherwise be confusing:

When directly addressing someone:

Let’s eat, Grandma!

With yes or no, unless that word is modifying another word:

Yes, please, let’s stop discussing commas. No, thanks, I don’t want to read any more about them. No thanks to me, this may have gone on too long.