the fastest way is to do situp twists or crunches. Right elbow to left knee and left elbow to right knee. Not only are you strengthening your abs but the twist exercises the rectus abdominis muscles which will pull your stomach in, strengthen your back, and improve your posture. Always remember that you cannot just exercise just one muscle group. If you want to strengthen your abs you also need to work on your back to help prevent back pain. Good posture is also important and diet can help some. I'm guessing from your picture you are male, an important thing to remember is men store excess fat inside the abdominal wall which means it's hard for just exercise to get rid of it. You'll need to watch your diet as well.
Well, when a boy likes a girl, they have a unicorn stop by and ... ANYWAYS little cells divide and form muscles all thrughout your body while youre in your mothers ... stomach! And you have abs! Just like that, you get to be born with abdominal muscles, no work needed
Leg-lifts after your sit-ups. Pull ups also helps. Warning the general rule is for every musle you work also workout the opposite to. Like if you work abs than do back raises. Hope it gets you a six pack like it did for me.