I would say a butterfly would be a good tattoo to cover the scar if you are a girl. If you are a guy I would say a snake slithering over some bones. Those are the tattoos that I would say to get for covering her scars on your tummy.
Be aware that your scar might not be a good candidate for a tattoo to be there. Consult with the tattooist before hand to make sure that the ink will settle there nicely and not be rejected. This can happen if it is a large, raised scar. Otherwise, any design would work.
Let it heal compeletely. The last thing you want is to get one and then it becomes permanently distorted because your scar was still healing up. Consult with tattoo artists for some ideas, it takes a greater amount of skill to cover a scar well as they vary in size and shape. Simpler also looks better than detailed.