Survivor since 2012
I understand perfectly that you are scared -- that's normal, but that doesn't help make it any less scary. First piece of advice that I give anyone is: DO NOT START SEARCHING THE INTERNET. There is a lot of inaccurate information out there and it may only make you more scared of the unknown.
You are doing the right thing: going to your doctor as soon as possible. Many lumps, especially at your age, are benign -- usually cysts. But your doctor will give you the best advice and based on his/her examination and any tests will help you decide what you need to do. Is your Mom around, or any other relative you can lean on for help with this? It would be good if you didn't have to go to the appointment by yourself. Of course, BTS is a big community and we're all here to help.
Just hang in there and take it step by step -- try not to get ahead of yourself. I know that will be really hard, but at this point the BEST thing you can do is get expert help. Please keep me posted on how it goes, and feel free to keep asking questions!