If you really want hard abs, then you have to undergo a strenuous regime at the gym. However, if you simply want to trim and tone it up a little, you can follow certain steps as given below:
Perform abdominal exercises. These include sit ups and stomach crunches. Start with minimal sets and then gradually increase them as your sustainability level rises. Also, core lift exercises like front squats, dead lifts also help in stomach reduction. Also, when not exercising, try not to sit around and include maximum movement.