Ha hes a way to train your dog how to sit.Say sit and push his back down to were he/she is sitting and say good boy or good girl and then say good sit boy/girl.
I don't really agree on this method. You should be able to teach a dog how to sit without touching him which is less stressful for him and he'll be more willing to do it.
Hold a treat above his nose and push your hand towards the back of his head so he has to sit naturally to get it. Give him the treat as soon as he sits (even if he doesn't sit long at the beginning) and say "sit".
To my point of view, there are 3 things a dog should know to be able to learn more after :
Always reward good behaviours and ignore unwanted ones. And above all, have fun with your dog. He'll love doing things with you and for you if he's having fun working with you.