Heya,
WHat kind of dog do u hv?
I would not hit the dog first if he does not understand wht u want.U can try this....(i hope it works) Since he does not understand, hv to start by creating a controlled environment...ie..leash him before he can make the mistake. Hv a long lead and leash him up. Open the gate and he will try to run out. Make sure u hv the leash in hand (controlled) and tell him a firm "NO". He will choke himself abit. Do this very often...and soon enough he should understand what "no" means..

Just my RM 0.02.. 

Your dogs will always feel excited when go out for a walk, so always make sure he is on leash, then command him to sit. Until he sit down properly, only u command go & start have fun walking ur dog! If he does not sit, do not bring him out until he sit. When time passes, he will understand that he can only go out through your command.

Of corse at times when u open the door, he might want to rush out, juz say a firm NO! + the above method, he will learn. 

you've got to train your dog - start by simple "no". say it sternly. don't fool around with the dog when you are angry - the dog can sense it when you are unhappy with him/her. it takes time. i too train my dog like realitydreamer - always leash your dog when going for walk. eg dog like to rush out and pull you along on the leash. just stop and say "no". after that wait for dog to calm down and settle down. ask the dog to sit, then when you are ready to walk give command like "go"/"heel". when dog pull ahead again stop. overtime he/she will understand to heel.