How can i gain fat to have a fit body shape am too slim to have a fit muscular body?

Eat healthy high: calorie foods. Eat 6 meals/day. Work on strength training to build large muscle groups. Workout w heavier weights w fewer repetitions. Sleep at least 8 to 9 hrs/night. Hydrate well w water. Foods to help w healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, Avocados, granola, trail mix, dried fruits, extra virgin olive oil, canola oil, protein shakes,>

Adding fat: will not generally contribute to a fit body shape. Even if you have a thin body type(ectomorph), you are never too thin to increase your muscle mass with a healthy diet and regular exercise including progressive resistance weight training.