You do not want to focus solely on cardio, because even if you are slim, then you will develop a layer a thin layer of fat around your muscles. You get rid of this by doing weight based exercises. Deadlifts are effective. But also try using the various machines available around a gym. If you do not have access to a gym, then simple crunches and sit ups can be effective to a point, but weight based gym equipment that focuses on the abdomen is most effective.
Also be sure to focus on diet. Half of your plate should be fruit and vegetables. A pack of game card sized amount of protein (eggs, chicken, turkey, beef, lamb, beans, etc) and the rest should be carbs like wholegrain pasta or rice, and a small amount of fat (healthy fats such as olive oil are better than butter).
In short: a good mix of weight training and cardio, along with a healthy diet is the best option. The abdomen is one of the hardest areas of the body to train, so do not give up if you don't see results easily.
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