what to do what to think?

Go to Planned Parenthood; they can help you out with the birth control.  Because you're under 18, you may need a parent or guardian's permission for the pill (but you'd at least be able to get condoms).  PLEASE PLEASE PLEASE, consider talking to your mom about this.  Yes, she might freak.  (In fact, I'd count on it, for a short while at least.)  She might also be imminently awesome.

The pregnancy tests in the stores should give you an indication of when you can use them reliably.  I suspect that they won't work right now, but if it makes  you feel better to try (and you have the disposable funds), go for it.

If you ARE pregnant, sit your mom down and say the following:  "Mom, Boyfriend and I had sex, and we were really stupid and did not use protection, and I'm pregnant.  I'm really scared and nervous and I know you're probably seriously disappointed in me, but I want to do the right thing and I could really use your love and support."

(Because frankly, a very small part of her in the back of her brain has probably been worrying about this since the day you first showed interest in boys.  Admitting upfront that you're scared, were stupid, and want to do the right thing won't make her less upset, but it will pave the way to getting her to help.)