chopsky

Firstly, I suspect the reason your tummy doesnt have good shape isnt due to the abdominals (which youre working with sit ups etc). It's rather due to your body fat. This is a common misconception that people have, hence why many end up doing 1000 sit ups every day and wonder why they never see their abs or have a decent waist.
Here's my advice:
1. Do more cardio (3-4 times a week). This will help raise your metabolism and burn fat. Look into a method of cardio called HIIT which is basically very intense cardio (bike/running/eliptical machine) for a short amount of time. HIIT is great for fat-burning.
2. Stick to the sit ups.
3. If you go to a gym, do exercises such as squats (and go to parallel with the barbell) which will work your core very well too. Search the net for proper form of a squat if you do decide to do it. Used properly, they're the best exercise you can do.
Do NOT underestimate the importance of weight training. It's the key to success.