Well, there's a few things you can do. We don't know how tall you are, but since you mentioned that you were already pretty skinny I will assume you are just looking to get some ab definition? Correct me if I am wrong
If that is the case, then your goal is most likely overall body fat loss. To reduce your overall body fat consider doing some or all of these things- these are just general guidelines, you should always check with your doctor before doing anything new
1) Eat 5-6 small meals throughout the day. Each meal should be 3-4 hours apart. Each meal should contain one protein and one carb.
2) Drink lots of water.
3) Rest properly. This means taking at least 24-48 hours between strength training the same muscles, and it also means getting at least 7-8 hours of sleep per night. Lastly, it means taking 1-2 days off from exercising per week.
4)Cardio. This should be done at different intensity levels and different session lengths. Consider doing a low intensity/long session, a high intensity/short session and a few medium intensity/medium length sessions.
5) Strength Training. Strength train each muscle 1-3 times per week. You should lift a proper weight and perform the proper amount of sets/reps for your strength training method.
It may just be a case of increasing your ab muscles too. I think the best ab routine is one that consists of exercises for your upper/lower/oblique and your rectus sheath (fancy word for the long layer of muscle underneath the upper/oblique/lower abs. It supports the spine the most, keeps our tummy from bulging out so much and helps support posture.)
I personally like doing a mixture of plank pose, reverse crunchs and 180 stabilty ball crunches. They're just my favs
Oh, one more thing your ab muscles are just like every other muscle. They need proper rest. So try not to exercise the abs every day.