You should maybe start with teaching your dog how to behave on a leash, if he has never been taught any behavior manners at all. Once he is learning to walk politely on a leash, he can be told to sit, when you stop walking, and pull his head up with the leash, and he will probably sit right down. Then you can pet him, and reward him for sitting, and soon he will understand the command to sit when you tell him to do that, and will automatically sit when you have him on a leash and stop walking.
Always be sure to let your dog know when he has done what you are teaching him, even if it is just a pat on the head, and a verbal "good dog !", to let him know he understood and did what you wanted.
Most dogs love to please us, and will try very hard to do the right thing once they understand what you are teaching them.
