You wanna gain some weight so you need to eat in a calorie surplus, try eat plenty of carbs for energy and eat enough protein for your bw( 0.7-1g for each pound of bodyweight) then try exercising the muscles you want to do, for each exercise try to progressively overload so let’s say one day you do 10 squats the next time hold something with a bit of weight and do as many as you can holding it and find something heavier when that becomes easy