I would start out with something non-strenuous since you have no muscle. Try walking several miles either every day or every other day (start with maybe a mile or mile and a half.. And increase your walk distance when comfortable). Also, get yourself some small weights and start doing some upper body weight lifting to get your arms and such back in shape. While your doing this, try buying some ensure drinks. They will have all the nutrition you need while you are still struggling with your eating disorder. Since your body has no fat and you are still having issues with eating, I would stay away from workouts that are too strenuous just to be safe.