Deciding to have a baby means deciding to be a grown up, so sorry but you have to act. It sounds like you are wanting to keep this child. It may be hard, but unless you get an abortion they're going to find out one way or another. Have a friend hold your hand if you need it, but you have to tell your mom. Tell her you're pregnant and your decision to keep, adopt, or abort. If you are worried she will force you to have an abortion or force adoption, talk to your boyfriend's parents. If you're worried she will kick you out of the house, again talk first to your boy friend's parents. I know many states in the US allow someone over the age of 14 to make their own medical decisions so in theory you won't be made to do anything you don't want. If your unsure, wait to tell her until you know.
For now, find a planned parenthood or similar clinic. They will make sure you're starting off to a good pregnancy by having the correct amount of vitamins and have the knowledge needed to have a safe pregnancy. That is the other thing you need to think about -education.
Educate yourself on this pregnancy. Books like what to expect when you're expecting are a good place to start. Although keep in mind sometimes information can be confusing and contradicting so if this happens go with what feels natural. Aka my sister in law drank herbal tea during her pregnancies. Some swear against it, some see it as a grey area (moderation is ok). But it's also good to know the don't eat foods because there are a lot of foods out there that can seriously harm a baby and something as simple as cleaning out a kitty liter box can cause a miscarriage. So educate yourself on this pregnancy- the library will have plenty of books you can check out.
The second thing you need to consider is your own education. You're 15 and I'm assuming you're not going to graduate early, so graduation day is a few years off. As you get closer to your due date you will need to figure out what that means for your high school education. There are many options out there so don't feel you have to give up.
It's scary and it won't be easy. But if you want to keep this child you need to become an adult and stand up for your health and the baby's health-because right now that's what matters.
sounds like you have a real problem being 15 and pregnant and you can't tell anybody. is your boyfriend under or over age first off?