Cardio first thing in the morning on an empty stomach is the best way to lose weight!
If you can do it
If you do it later in the day your just burning the calories from the food you ate during the day.
If you work the, say treadmill first thing in the morning you haven't eaten anything for the last 8 or so hours because you were sleeping. So, when your body sees that there are no carbs to burn, it goes directly to stored bodyfat. And stored bodyfat is the fat that is on your body, which is the fat that you want to lose!