How much weight could I safely lose in five months and what are some of the best ways to do so?

I really want to lose about 70lbs. but I'm pretty lost as to how I should go about doing so. I know I need to exercise and eat healthier, and I've started doing so as best I know how by eliminating soda and most all junk foods and I do thirty sit-up nightly. What types of foods are the best to eat and what should I avoid? Also what are the best exercises to do? It's pretty cold outside now so, indoor things would be ideal.