I'm assuming your dog is about the size of a cat

I showed cats and am pretty good at grooming long and short hairs.
Set up a grooming place/time and be consistent. Do NOT let the dog decide on grooming rules. YOU decide. Teach the dog to stand or sit when being groomed. Even if you have to use two people at first your dog must be taught manners. Try holding on the scruf of the neck (works in cats) and comb/brush with the other hand. If you have a long haired dog, you really should be using combs and not brushes as you won't get to the undercoat with a brush (unless its got teeth/pins).
Watch at dog shows how they get the dogs to stand and be groomed - this is taught as puppies. You have a young dog, take control of the situation now.