You can probably find the nutrition information for Jack-in-the-Box on their web site. If you ask at any of the restaurant locations, they probably are required to give you the information. Since you are on a diet, however, you should know that fast food is probably not your best choice unless you are on a high-fat diet.
If you are on a diet, Jack in the Box may not be the best place to eat. However, if you must, find the healthiest foods by looking on the Jack in the Box website, they list most of the nutritional facts for all their foods.