What Causes Severe Hair Loss At The Age Of 20?

sir,my age is 20. i am losing hair for the past 6 months and my hair became thin mainly on the middle of the head (front)..the sides are normal..back side also normal. also i have white hair which came at the age of 10.....at present new hair is not appearing on the middle of the head... please suggest me the preventive measures for stopping this hair fall and how can i get my hair grow back normally