When to do cardio?

To burn fat is it best to do cardio first thing in the morning or when?

Yes in the morning before breakfast.

Doing cardio first thing in the morning, on an empty stomach, before you eat anything, is by far, the most effective. Whether you run, jog, walk, swim, jump rope, bike, etc., doing it first thing in the morning is the best time to do it for maximum fat loss. Why? When you do your cardio workout you obviously burn calories. Lets say you wake up, eat breakfast, eat lunch later, and then do your cardio. All you will be doing during that cardio is burning the calories of carbs of the food you just ate. When you do it in the morning, you haven't ate anything for the past 8 hrs. or so. So when your body sees that there are no carbs to burn, it goes directly to stored fat.