Why am i always tired?

im 18 and am 200 lbs (pear-shaped)...anywho im always tired im mean im trying to eat right and take my vitamins and even started drinking that god aweful breakfast essentials stuff!...plz help theres no reason for me always severly tired!..im young and i get my 8 hrs a day and i work and go to collge but ive had this problem for years! help i just cant get threw the day! thanks!