Hello,
there is no way around this - you need to see a doctor and determine what's causing this. It could be either stomach or gallbladder problem, but since as you described it it seems that you're nauseous whenever you eat - this also might be some kind of food intolerance. Not to sound too preachy, but there is a reason junk food is called just that - it has too much sodium, it's often too spicy, acidic, fatty and it has all kinds of additives that can injure your stomach lining.
What happens if you try eating fruit or vegetables? Does your stomach react the same? Since you're only 13 it's very important that you get all the nutrients - vitamins and minerals you need because you're developing. IS there any food you can eat without problems?