Rhea's answer is pretty good. I just wanted to expand on #4: track your progress. Seriously, this is massively motivating. Going back through a workout journal and seeing the progress you've made is amazing. As always, there is an xkcd comic for it. If you are running in particular, an app like RunKeeper can do a lot of the tracking, exercise planning & coaching for you, including recording distance, pace, intervals, plotting the run on a map and more.
Having said that, running isn't the only way to get in shape. If you don't enjoy it or can't get into it, consider other physical activities instead. Regularly play sports. Learn martial arts. Learn to climb & boulder. Try swimming, biking, or rowing. Take up weight training or Crossfit. Find something that you enjoy and can do consistently and the fitness & health improvements will come along for the ride.