You poor girl. Because you're hiding it, you're getting the support you're going to need.
You might think just having baby, and telling the hospital you “don't want it, bye now,” that is end of story. But it's not. It won't ever be.
Periodically, you're going to think about your baby. You'll wonder where he is, the family he's with, what he looks like. On his birthday you might cry a little and wonder if he's walking yet and saying Mumma, Dadda. Every year on that day - you will remember him; wonder what he looks like. When you're 19 and he's 4 years old, you'll think about him starting school. Over the years, you'll see babies and little boys and girls around the same age as him and wonder if you ever saw him whether your heart would know him or whether you'd walk right by.
Do you see where I'm going with this? It's never ever just ‘over’. I just want you to prepare yourself not just for his birth or walking away from him but for the rest of your life without him. Be aware and be prepared.
Fifteen is young to raise a baby, I totally agree. You haven't finished school, been to college, had the experiences you'll need to become a responsible adult.
What I WILL say to you is this: you are missing out on the best support you may get, right in your own house. TELL your mother. Don't hesitate any longer. She may be upset at first but her very next instinct will be to protect and support you. Your parents may want to raise him. The benefit of that is that you will always know where he is, what he looks like and that he's in a safe, loving home. There are all sorts of put-together families out there today, and grandparents raising a grandchild is more common than you might think.
If on the odd chance your parents are unable or unwilling to raise your child, then they might be able to find another willing relative who would happily take him. Then, he still stays within his family.
If noone can take him in, then ask the baby's father's family. For your baby, it's the other half of his family. If nobody in either family can take him, that's when your parents will help to get him adopted. Let them help you. You don't have to do everything on your own.
If your parents refuse to help you, then speak to a Teen Pregnancy Support in your area. There are phone numbers online available. But only use this AFTER you tell your mum and if neither family will help you. I think you'll be surprised how protective your mother will become. Mums are usually very adaptable people!
I wish you well.