Make sure you get yourself a decent pair of shoes if you start running or walking so you don't trash your feet.
Push-ups and Sit-ups are basic. But, effective. Get up to doing 100 and you will be amazed at the results. Add some squats for the lower body. Hindu squats if your really motivated!
If your weight is low, make sure to increase your calorie intake when you start exercising. Your body will need it. I had problems putting on weight when I was 18. That changes in your 30's! Adding a few healthy snacks in between meals should help.