i recently computed my bmi and i found it that i am normal yet my bmi is almost near to the value of underweight. physically i'm lean (i'm a guy), an i'm wondering how can i gain weight easily? please help me i really want to gain weight..
also tips for working out? my mom said that if I worked out earlier, I'm not giving time for my muscles to grow by themselves and maybe I should only start working out at an age of some 20 and above? I'm 5'2" and 17 years old.

what ordinary exercises may help in the weight-gaining process?