first time dog owner

Question
Brindle pitbull pup... Recently stopped nurseing from mother. About 3 months old.. What's the best choice of food to start puppy on?

Answer
Any of the common brands should do.  I know Pro Plan, Iams, Purina, and Science Diet are good because they are the choice of the professionals managing service dog schools.  Their selection is based on facts, not the speculation, anecdotes, junk science, marketing hype, and general distrust of large companies that is the basis of most opinions on dog food.  The schools testing of different diets isn't available, but after testing different foods, they feed common brands.  

How much you feed may be much more important.  You should be feeding a puppy chow now. Switching to an adult chow for the larger breeds at 4 months, slows growth and helps develop sturdier joints.

Your dog definitely should be narrower at the waist than the hips and chest. You should be able to easily feel the ribs, but not see them. Each dog is different. Standard recommendations are a good place to start, but each dog must have its food and exercise adjusted to its individual needs. Here is a link to a good illustrated guide, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx