What does every 18 year old need to know?

They are now able to legally make bad decisions regarding their health. Look at the people who are your grandparents ages and find the ones who are healthiest and emulate them. The ones who drink, smoke, eat badly or don't exercise do not do well. Also, realize you are not immortal. The number one killer in your age group is still preventable accidental death like drunk driving or emulating Jackass stunts.