Look at the post in this forum on exercise fuel.
In a nutshell, the important points for fat burning are these:

You burn fat best during long, medium intensity cardio.

You build fitness faster by adding in some small sections of highly intense cardio.

The more fit you are, the longer you can exercise, and the longer you exercise, the more fat you burn!

The more fit you are, the more efficiently you burn your fat stores.

To make sure you have lots of energy for your next workout, eat your carbs after your workout and/or use a little dilute sports drink during your workout.

Make sure you are also doing strength training twice a week on top of your cardio...that builds muscle, which burns calories even at rest.

Therefore, you best and quickest route to weight loss is to eat sensibly, but count calories, do cardio three times a week, and do strength training twice or three times a week.