Exercise - you should be getting a good cardio-workout at least 3-4 times a week. That's getting your heart rate up and keeping it up for 30 minutes. You should also do strength building exercises a few times a week as well.
Eat Properly - skip the junk foods and energy drinks! Eat as much natural, whole foods as possible. Try to avoid foods that are overly-processessed, added-to, and sugary.
Get enough sleep - adults need from 6-8 hours of sleep a night to stay healthy. Kids and teens need more like 10!
De-stress - Learn healthy ways to manage the stress in your life, like meditation, tai-chi, or exercise.
Think healthy - learn how to use healthy thinking to solve your problems instead of worrying, being anxious, getting angry, and obsessing over things.
Get rid of drugs - this means alcohol and tobacco along with any drug used illegally (that means abusing prescription drugs, too)! If you can't quit, get help now.
Vitamin up - if you're not eating properly, or don't have time at work to make healthy choices all the time, take a multi-vitamin daily. It might be less effective than eating the right foods, but it's better than not taking one.
Make friends - studies show that people with healthy social lives (not the kind where you go out drinking/smoking/drugging!) are healthier than people who just stay at home alone.
Get religious - studies have also shown that people who practice any form of religion (pick whatever one you want!) are healthier than people who don't.
Just try! - anything you do to try to be healthier is better than doing nothing! Just asking this question counts as trying. Get out there and make a few changes and get healthy, my friend.