well, since the advice snobs on here hate this question, I will help you out.
Abs is 2 things and 2 things only !!!
1. Genetics, as with anybody part........ this is a huge factor, some people are just not meant to have really ripped abs.
2.Low body fat, everybody has abs......just some people have a layer of fat covering them. want the best abs that your genetics can give you, major cardio and a clean diet.
other then that your abs are a muscle like any other one in your body, you can make them grow bigger and thats it, but the fat is what's making them less noticable.
hope that helps