Sit
There are many different ways to teach the SIT command. There is neither one right way nor one wrong way. All of them work. Some work better than others for different dogs.
Here are two of the most common methods.
Lure method
This is probably the most classic of methods. Start with the puppy in a standing position either beside you or in front of you. To get the puppy to sit, take a small treat directly to the dogs' nose and smoothly lift it up and back, just a few inches above the puppy's nose as you say "SIT". The head will follow the food up. Once the head goes up, the rump may drop and your puppy will be in a "SIT" position. If the rump does not drop to the ground, then you can try moving the treat just slightly higher, but not so high as to make the puppy jump.
If the puppy sits, then you may praise the puppy "good dog" and reward it with the treat. For a number of dogs, this method can work quite well. As you practice, it is important to vary positions, so that the puppy learns that "sit" means to get his rump on the ground, regardless of where he may be in relation to you.
Hands-on method
Another method to teach sit is a little more formal and hands on. Your puppy may be on or off leash, but must be wearing a collar. Have the puppy standing next to you. Grasp the collar gently with your right hand. Place your left hand on the puppy's rump. As you say "SIT", simultaneously lift your right hand slightly to elevate the head, and slide your left hand under the rump to tuck the rump down to the ground. Do not press straight down on the rump as you can injure the puppy.
Praise and reward your puppy when he sits. Note: With very young or wiggly puppies, a hand on the rump may cause the puppy to turn and look to see what touched him. If this is the cae, you may have better success with the lure method.