Diet is extremely important for every pet. We recommend feeding a premium food that uses human grade ingredients, is not loaded with grains and contains no chemical preservatives. Many lower quality foods will contain split grains. You'll see Chicken, Lamb, etc., but then you will see three different types of corn or other grain that will outweigh the meat, making the food grain-based rather than meat-based. Dogs are carnivores who can do well on an omnivorous diet, but a grain-based diet is not optimal. Chemical preservatives to stay away from are BHA, BHT and ethoxyquin.
As far as canned food, again you will need to find a high quality canned food. Most of the premium foods also sell a canned food.
Never feed your dog cooked bones. Cooked bones can splinter and cause serious damage and even death. Many people give raw marrow bones for chewing. Check the size of the bones. There have been dogs who have gotten their lower jaw stuck in the bone. It should be big enough not to swallow, but make sure the center is large enough or small enough that this cannot happen.
There is also the BARF diet, which is "Bones And Raw Food". For information on feeding a raw diet, Dr. Pitcairn has written a book, Ian Billinghurst and Kymythy Shultze have written books, among others. You can search online for these books or find Pitcairn's book in the JRTCA's book store. You can also search online for more information on the BARF diet and homecooking diets. Please be sure to get one of the books. Although many dogs do very well on these diets, they do need to be properly prepared and without proper knowledge of how to feed this diet, it can cause more harm than good.
There are no hard and fast rules about amounts to feed since there are many variables that come in to play: age of terrier, climate, exercise, etc. Feed the recommended amount listed on the bag and adjust accordingly. You should be able to feel the ribs (but not see them) on a fit Jack Russell. Structure (heavy or light boned) will also play a factor. As a general guideline, a 12" dog should weigh approximately 12-13 pounds.