To lose weight anywhere it's best to work out your entire body. You cannot just focus on crunches and expect the fat to go away.
There are also a lot of diets that can cause your stomach to bloat. Mainly too many carbs (grains, starches, sugar).
Cut those, and also carbonated drinks if you're prone to drinking them, and do a decent workout every 3 days or so, (cardio and muscle development, doesn't have to be weights, can be a sport) and your tummy should flatten out.
Generally from what I've read and studied. When it comes to losing weight. You can't necessarily focus and just one part of the body. Especially when it comes to tummy fat. If you eat things that are low in calories and give you tons of vitamins and minerals, then exercising is the easier part.
From my experience when you start doing that, the fat melts away. Generally it's in the tummy, face, thighs and arms where it really starts to show. Good luck! That's what I'm working on too!
Here's the deal; although you don't seem to be male, the tummy area holds fat, the tummy area also has receptors, these receptors tell when to store fat and when to burn it
Men and women in the stomach area have less of the burning receptors, this is because the tummy is a great area to store that extra energy that is known as fat.
Now on to the next thing. Fat is distributed equally* Its not BURNED equally, because the stomach area has less receptors, its the first area fat is put ON and the last area to GO.
Now, inorder to lose fat you need to intake less calories than you burn off. simple as that. Mind you during this process you must still maintain the nutrients you need;
Cardio (like running) is a very good option in order to burn calories, the only issue is that you can burn a lot of calories but AFTER you're done you stop burning calories, its not a continuous process.
The second thing you can do is light weights. DO NOT freak out, seriously. My assumption is that you're female and most of the time theres a "oh no, I don't want to become some kind of bulky blah blah blah." This isn't the case. You wont become some kind of body builder instantly,
instead the goal is to rip and repair muscle. During and AFTER a workout you are burning calories because it takes energy to repair that muscle. also, muscle weighs more than fat, so when you are looking to lose fat
Dont look at a scale. That is a BIG big mistake, a scale wont tell you if you have fat on you, only you can look and see. Muscle will replace fat, your weight won't change much (in fact you could gain weight) and look fantastic.
Hope this helps,
To Address the sit-up/crunch thing
The core is a lean muscle area, you don't burn too many calories by working this area out, also improper form can cause bumps in the muscle fibers which will grow under your fat, which if done often enough improperly enough will make you tummy area look bigger. Abs are made in the kitchen, not the gym. Those are abs, if your looking to flatten, same idea.