Cardio! I suggest running up hills (increased incline on the treadmills if you want to go to the gym), stair masters (equipment in gym) as well as other machines that the gym have. Gyms tend to vary from what they have but for the most part most cardio machines have certain workouts for thighs/butt that you can choose. Try to push yourself. If you aren't having trouble and sweating then its not going to work to burn off what you want to burn off.
1- Your diet- Eat anything that is not made by humans. Things that are grown are the best. Avoid all fast foods. Then when you eat, make sure you leave the food before you get full. Three meals a day is your best bet (Breakfast, lunch, dinner). Limit snacks to fruits and veggies.
2- Excercise- My personal opinion is that for anyone trying to build mucle or lose fat, the park should be their gym. You can do so many different exercises. Cardio can be done by running laps in the field. You can do pullups on the monkey bars. Pushups on the ground. Dips. Situps. Stretches.
A good fat burning excercise is to go for a run. And every time you reach a stop sign, get down and do pushups or situps and the continue to run.
If you follow this I'm sure you will be SEXY in no time haha
Good luck and make it a priority to live a healthy, full lifestyle.