Hello Sir, I Am A Boy Age 19.my Weight 50.i

Hello sir,
I am a boy age 19.my weight 50.i want to gain my weight aximum 65 kg as fast as i can.please help me doctor.one thing i sometime do exercise like puss up etc but not regularly.i am very depressed about my condition but one more thing i am not weak like other thin people.i must walk 5 to 8 km regular.nd take 3 time s meal how much i can so doctor please help me please..Ans fast.