The sit command is a very good “default” behaviour. It is a sign that your dog is being attentive and is in a somewhat calm state. There are many ways to teach this command and your dog will learn very quickly if it is re-enforced at the right time.
Start in an area where your dog does not have many distractions and position them with their rear near something solid so they cannot walk backwards too far. Get the dog’s attention with a treat in your hand by allowing them to smell or lick your hand.
With your dog’s attention on the treat, drag the treat just over their nose, moving it back towards their neck. Most dogs will try and follow the treat and drop their rear in the process.
If it is not dropping its rear, gently apply pressure to it, and as the dog starts to drop the rear by itself, or as it touches the ground via your pressure, reward instantly with praise and the treat.
Keep the treats coming and try to keep the dog in the sit position until you tell the dog it’s “free” with some sort of movement to get the puppy out of its sitting position. There is no point having your puppy sit if it just gets up and leaves or jumps up on you whenever it feels like. Make sure from the first exercise that the puppy only gets out of a sit if you say that special word: “free”!
As you repeat this method, use the command “sit” once at the beginning of each repetition. Soon they will make the association that when you say this word, and they do eventually sit, rewards follow!
If your puppy comes up to you and sits, looking for attention, by all means give the puppy attention. Remember, when your puppy is sitting it is not jumping, biting or doing anything else undesirable.