Sit, is the first and most basic command to teach. Actually, it is quite easy. As always, you will need your supply of tasty treats and 5-10 minutes a day to practice.
While your dog or cat is standing in front of you, show him the treat. Hold it just above his nose. When he sees it, move it backward toward his bottom. He should naturally follow it with his head and to do so, will put his bottom on the ground. As he lowers his bottom, say sit. When he sits, reward him with praise and a treat. Repeat.