You're not going to make any significant changes in a month, BUT you can start working out now so you can reach your goals eventually. How long that takes depends on exactly what you want to accomplish and how diligent you are with your workouts and eating, to reach your goals.
If you're not already pretty close to being what you consider "in shape", there's nothing you can do in 4 weeks that is going to make a huge difference. You might workout and start toning up, but you're not going to be able to get really firm or add much muscle at all in that short a period of time. It could take many months.
For belly fat, eat clean (avoid processed foods). But make sure you're eating enough, because you don't want to lose weight if you're already underweight. To build muscle you need a calorie excess (to lose fat you need a calorie deficit).