I defeted Misty in Cerulean city right now and I cant go anywhere becouse I dont have the move "cut". I also can't go back !
You can get Cut in the ship located at Vermillion City.
Then, teach it to one of your Pokemon that can learn it (Sandshrew, for example, can learn it).
Hope I helped!
Source: Experience (I had the same issue first time I played the game)