Begin with the command sit! It's a simple command that you can also practice easily by first doing fun games and when your puppy feels relaxed and at ease you will notice they will naturally be more inclined to sit down without you saying a word.
As they start to sit (putting their bum on the floor) say the word Sit and follow with a reward. What this will do is help them understand that when you say the sit command in the future, that they should associate it not only with the action of sitting but also of feeling calm and relaxed.