It depends on your "frame." Some people carry weight better than others. But you're well within the healthy weight range for your height. If anything, you're on the "thin" side, so there's no reason why you should need to lose weight. I'm sure that you look great. That being said, if you want to tighten up your stomach, obviously any ab exercises are good, but people forget that the most effective way to burn fat is through cardio. So, I'd say that your best bet is to work on your core strength through planks, crunches, etc. but to put more emphasis on boosting your metabolism/ability to burn fat through running, swimming, biking and other forms of cardio. Get your heart rate up, and your tummy flab will go down :) Good luck.
You can lose stomach fat by losing overall body fat but you can't target fat loss anywhere. It seems like your a healthy weight. I prefer a slightly thinner girl depending on how you wear your weight but a lot of guys wouldn't agree. You would still be at a healthy weight at 145 and even at 135 but I wouldn't go lower according to the BMI calculator.