My uncle has cancer of the of the colon due to poor diet. we have no cancer history in the family but i've been wondering if this is hereditary?

Unlikely genetic: Just one member of the family is not enough to call it familial cancer , 75 % of colon cancers has no family history , so don't worry , you are 21 and if no symptoms , 20 years from now go for regular screening colonoscopy , you will be fine. Yes diet plays a roll but unlikely it is the cause of cancer , develop good dietary habits, eat lots of veggies, fruits, less re meat , will be fine.