It's usually the first command that everyone teaches to their dog. I have included a video to show how to teach the behaviour without having to touch the dog.
You start out by luring the dog with a piece of food. The food is raised over the head of the dog. Trying to look at the delectable tidbit will cause them to place their bottom on the floor. The have just learned the behavior without very little coaxing.
After they learn to sit with the lure you add the command sit just before you lure them to a sit. Then the next step would be to phase out the lure and leave just the command. You can also use a hand signal with or without a verbal command.