Let your dog get into the habit of sitting beside you on your left side, which is the purpose of this training. In this way, the dog will be more close to you, as well, it's propitious to train.

What's the correct sitting posture? Hind legs bend and forelegs should be straight, the neck and head face toward the front. Now, let's train your dog to sit. Hold his collar with your right hand, and press his lower back with your fingers of left hand, let the dog sit down. As well, you should say command "sit" ceaselessly. Repeat the training again and again, the dog will grasp the action soon.

when you train a puppy, you can give him food first, after some time, puppy will get into the habit of sitting posture naturaly.