I'm going to make a few assumptions: limited space, limited $, slightly overweight (or more), beyond 20 and you're looking to lose weight and improve overall health....
Diet is key to losing weight, start writing down everything you eat for a week (everything) and review the calorie intake each day (use http://www.acaloriecounter.com/ to determine the calories). Use this daily caloric calculator to give you an idea of how many you should be consuming (http://www.healthyweightforum.org/eng/calculators/calories-required/) - remember, calories burned, consumed and required are all estimates - you need to adjust your diet to fit your personal needs and body requirements.
A great way to improve your health is general, simple exercising that you can do every day with little impact to your current schedule. Take a look at Fit Deck (http://fitdeck.com/fitness-bodyweight-only/) body weight exercises that can be done in minimal space, with no required equipment and about 30-45 minutes a day (5 to 6 days a week).
I also ALWAYS recommend finding an activity that interests you and that gets you out and active. Good luck.