How do I get rid of body fat if I'm already skinny?

Even though you may be at a normal weight, you can still have excessive body fat in some problem areas such as your abs, thighs and arms. Since you're already at a normal weight, you shouldn't cut your calories too much since doing so will cause you to become underweight which isn't healthy.

Calorie Balance

What your goal should be is to eat the same amount as you always have but increase your exercise level. Since you aren't trying to lose a large amount of weight, you should increase the amount of weight training you're doing. This will build muscle while burning calories to get rid of the fat you have laying around.

Build Muscle

Building muscle allows your body to burn more calories at rest which will help you in further reducing your body fat percentage. You can also increase the amount of cardio you're doing by a bit. This will also burn a few extra calories to speed up the process.

Food

Apart from the exercise, you can also change the type of foods you're eating. Meals that are high in sugar are digested quickly which will add to your fat storage. Eat small, frequent meals that are low in sugar and high in complex carbohydrates.

Spot Reduce

Remember that you can't spot reduce, so doing hundreds of sit ups won't do anything to reduce the fat covering your abs. With proper nutrition and exercise, the fat will start to go away, and you'll have a more toned appearance.