If you have never put an AKC agility title on a dog, then you belong in Novice A. If you have earned ANY agility title on ANY dog, you must enter Novice B. For example, if you received a Novice Jumpers title on your dog, you would enter Novice B Standard for your next trial.
If you are entered in Novice A Standard and JWW and earn a Standard title, you may move up to Open Std. You must stay in Novice A JWW for any trial that has closed. For any trial that is still open, you will now enter Novice B JWW.
After you have earned your first AKC Agility title, you will never be in Novice A again. Your next dog will start in Novice B and any new classes you start will also start in Novice B.