Usually getting a six pack takes a lot of hard work and dedication. Seeing as you already have a hard, flat stomach it shouldn't be that difficult. You should start with making sure you have a healthy diet, eating junk will only add weight making it harder to gain a six pack. Next is exercise! You will be doing a lot of that. Doing sit ups and push ups regularly will greatly contribute to gaining muscle. Any other exercises that focus on the abdominal area would be good. Hope I helped and good luck :)
To get six pack abs, you need to combine a routine of exercise and a nutritious diet. Workouts should consist of cardios and keep a steady metabolism. Take whole grains in your meals, plenty of water and do crunches as well as sit ups as part of your exercise. Read more details at www.wikihow.com/Get-Six-Pack-Abs.