If you are not in shape, then I suggest you start a light work out regime. If you go to hard too fast, you will get burnt out and not stick with it. I suggest you do about 15-20 minutes of cardio. Follow that with about 10 minutes of stretching (of course you need to stretch before the cardio as well). After stretching, do some light arms (5-10lbs for 20-30 reps per muscle group). Do this about 3 times a week for the first three weeks. Then up it to 5 days a week for 3 weeks. After that, increase the cardio time, number of ab reps, but not the weight or reps on the arms.
You will also need to adopt a very healthy diet. Different diets work better for different people. For me, a mostly vegetarian diet is the best. Others are better off with a paleo diet, vegan, or general balanced diet. This is a trial and error process that you will need to do for yourself.
REMEMBER: This will take time. It should take at least 1 month before you see any lasting changes so don't get discouraged.
(for what it's worth, I used to be a personal trainer)