You can't lose weight from only one area of your body - like your bum for example. When you lose weight, you will lose it from your whole body.
Although it may seem that you are losing weight from one area (because of your body type - I have a pear shape and my upper body seems to lose weight first and my heavier thighs seem to lose weight last), you are not. Because some parts of ones body are prone to store more fat because of the way we are built, it seems like we are able to lose weight from only one body part.
So to answer your question, you will have to lose weight (not only from your bum), but from your whole body. And the only way to lose weight is to consume less calories than you burn or to burn more calories than you consume.
You can do this with either two ways or a combination of the two - eat less or exercise more, or do both.
To eat less you will have to watch your portion sizes and avoid sweets, pastries, sugar, fruit juice and unsaturated fats. You can also bulk your food up with lots of veggies and/or fruits. It will full you up without having too much calories in them.
With exercise the best way to burn of excess calories is do to some cardio. But the problem with cardio is that too much cardio can start to burn away your muscles and the calories burnt with cardio stops when your cardio activity stops.
To solve the cardio dilemma, you must pair it up with weight training.
Weight training builds muscles and it also has the ability to totally reshape one's body. It can make a pear shape (like me) look more firm and less wobbly and it can make a saggy bum look more toned and lean.
And the best thing about weight training is that the more lean muscle mass you have (from training with weights) the more fat you burn and you even burn calories in your sleep (unlike cardio where the calories burnt stops).
The exercises that topgunjager suggested will help you to shape your bum. Another great exercise for reshaping the bum is lunges.
Good luck with your efforts.