When singing, most people who have not taken lessons sing only with their throat and their upper body. It is much like they are talking. But we know that when singing, we need more volume than just our speaking voice. That is where our diaphragm comes in. It is located below our rib cage. There is one exercise I want you to try so you can "feel" it. Just say the word "hey" several times. Don't be shy--say it loudly. Can you feel that tightness under your ribs when you say it? That is your diaphragm tightening up.
Now, take a deep breath, and you will also feel it. We use our diaphragm for breath control and pitch control. When singing a high note, a deep breath and tightened diaphragm will help you reach that high note easier. A vocal teacher will help you achieve this. For now, just practice saying "hey" so that you will begin to notice your diaphragm when you sing. Then, when singing, try to get the same tight feeling when you increase your volume and sing those high notes. This is one step in the process of learning to sing well.