Any exercise helps, but I think if you build more muscle, you burn more calories. You have to burn more calories to burn fat, I think.
For the amount of calories I usually intake (I love to eat!), I typically have to exercise for 30 minutes to 1 hour at least 3 times a week to stay in shape. Weight/resistance training, combined with moderate cardo really helps. I also try to do yoga/pilates once a week — I think it elongates muscles and helps me look more toned.
I also like to eat wholesome foods (whole grains and good fats) to give me energy and keep me satisfied longer: salmon, nuts (especially almonds, peanuts and cashews); flax seeds and flax oil (in yogurt or cereal), sprouted grain bread… Go for super foods, too: blueberries, turkey breast, avocodos, nonfat yogurt and lots of veggies, especially the green ones. Avoid white starches and carby snacks: bread, rice, pasta, potatoes (sweet potatoes are OK)… I still eat chocolate and fast food once in a while. You gotta live, too! Just keep moving and pick up the weights on a regular basis.