How to get rid of concave belly?

Ab workouts should help. Try things like planks, crunches, and sit-ups. I honestly wish I had your problem though. lol. I'm a size 10-12 too but mine is flat unless I suck in, then my ribs stick out really far. I wouldn't worry about it if I were you, but if you're bent on changing it, then the exercises above should help. (: