How to lose belly fat as well as gaining weight at the same time?

Hi, I am a skinny guy 16 Yrs old. My Height is 178 cm (5' 10 in) and my weight is 54 Kg. My BMI is 16.9 which is not good. I have some belly fat and I want to lose it as well as on the other hand i want to gain weight uptil 60 Kg. I dont do any physical activity. I have started to go daily on treadmill in the morning before breakfast for about 45 minutes and I work with 6 repetitions with 12 Kg for 15 minutes with alternating hand. like i do 6 repetition with right hand and then with left and so on. I have also increased my diet! I am hoping to lose my belly fat and to improve my muscles please give some more advices. Please tell me what is the best time to work out with weights? before meal or after?