Hi there,
I think you might be getting a little mixed up with what you want exactly. You say that you want to lose "weight" but that means you might want to get rid of some muscle too. You then say that you'd like to "shape" (I'd call it tone) your body a little which implies you want to build some muscle. I'm not trying to pick at your question - I just want to clarify the point for you.
To answer it in the best way, I am going to look at it as if you want to get rid of fat AND build muscle at the same time. If you're looking to "trim" and "tone" your body, and espectially because you suggest you are not that overweight, I would suggest you start performing more intensive aerobic exercises such as jogging. I wouldn't think you need to change the whole way you eat or go on a diet because if you're wanting to build muscles - it's better to do it all with exercise.
Exercises like jogging won't exactly "build" your muscles on your stomach but they will help reduce the extra fat around there. Depending on how fit you are, you should go on daily or bi-daily jogs of around a mile. To build muscles, you should concentrate on anaerobic exercises such as sit-ups to help build muscles.
These muscle-building exercises will give your muscles the toning and training they need to grow and become more prominent. To start to see a real difference with sit-ups, you should be looking to do no more than 100 every day (more than that will often hurt your muscles).
If you stick to doing those two forms of exercise - jogging and sit-ups, you should see real improvements in a week or two. Remember, you should do them as often as you can (I.E every single day).