I need to tone my stomach?

Okay so I've always weighed around 95 to 98 pounds, I eat regularly I just have a fast metabolism I suppose. I am pretty thin in my arms and legs but my stomach I am not comfortable with. I want a flat tummy, not a pudgy one. I feel self conscious in tight shirts and I hate the way I look in bathing suits, etc. So I want to work on toning my stomach and getting a flat tummy before summer. I have never religiously exercised before so I have no idea where to begin. It would have to be something I can do from home and something I can work into my daily routine, you know? The only thing ive been doing is crunches but i would like other ideas. So any hints or suggestion on what I should do would be great!