ANDY'S ANSWER: The symptoms can vary with each girl, but some get cramping in their abdominal area and others may get a lower back pain.
You may feel like your abdomen is bloated or just generally feels uncomfortable. You may also notice having more gas than usual, maybe diarrhea, slight headaches, or perhaps even feel a bit nauseous. Some girls experience more acne around their periods.
Other symptoms could be a white discharge on your underwear or tenderness in your breasts. Some girls experience changes in their emotions, perhaps becoming more sensitive or irritable than usual. (As you get older and learn to recognize the hormonal influences, you will be better able to keep your emotions under reasonable control.)
It won't take long for you to recognize the signals your own body will give you each month once you start to menstruate on a regular basis. Some girls may experience from mild to substantial discomfort with their periods while others have hardly any symptoms at all.