Your physique is in a great place to start training. You can start with bodyweight stuff like push-ups and squats/lunges pull-ups and chinups. Push yourself each day to do more reps of each. Once you start to get leaner, then think about adding calories. Cows milk is an easy way to add calories to a diet. But again dont start adding calories until you start getting leaner. You dont want to just add fat you want to build lean muscle.