Dear Lybrateuser, you can gain muscle mass & have a fit body by consuming a well balanced diet with more of protein rich foods & doing regular exercise - have more of fruits, vegetables including green leafy ones, whole grains instead of refined ones, protein rich foods such as eggs, lean meats, milk & milk products, tofu, beans, lentils, sprouts, peas, nuts such as almonds, walnuts, cashews, pistachios & peanuts, also seeds such as sesame, chia & pumpkin - include a protein rich food in each of your meals & also as a snack in between meals to boost your protein content - do regular exercise like jogging, swimming, cycling, pushups, pull ups, crunches, squats & moderate weightlifting for 15-20 min twice a week.