To get started, anything you do consistently will be an improvement - just so long as you don't push yourself to the point of injury. Trying to find the perfect program will often cause more problems than just beginning a modest regimen of some light calisthenics or weight training.
One thing I can recommend is
FlowFit, which was referenced in another thread. It's a free DVD (you pay shipping), but there are thousands of us who paid $40 for it in the past and continue to use it. It uses bodyweight strength and conditioning exercises in an intuitive progression that will get you in shape without risking injury or overtraining. It's also simple to do in a small space and requires no equipment.
Good luck.