Do some high intensity Cardio based workout. You'll burn a lot of calories, in a safe and achievable workout if you try and aim to stay in your cardio zone (65-85% of your maximum heart rate) for as long as possible.
For example in a 1 hour Cardio session, working at 75% of your heart rate, you could burn 800 calories, 280 of which would be from fat.
For girls it would probably be less, more around the 600 mark, and 210 of which would be from fat.
Typically, working at 75% of your maximum heart rate, of the calories you burn, 35% will be fat burn. If you work at a lower intensity, say 50% of your max heart rate, you will burn more fat calories (around 50%), but overall you will burn less calories, so you will burn fewer fat calories, even though the proportion of fat calories are higher.
My advice would be to invest in a heart rate monitor, Polar do some really good ones which you can pick up for around £55 online which give you loads of stats, or lower end ones for around £17 from places such as Decathlon, but provide you with less info.
Then go for a run or something, or sign up to a cardio based workout session and aim to workout in your cardio zone for as long as you can!
(Maximum heart rate = 220 - your age)