Go on the NHS website and search around and find the nearest sexual health clinic to you. I know you may find it awkward and scary going but the staff there have seen much, much worse and they really just want to help you. The clinic can also screen you later on for pregnancy if you are afraid of going to the shop and getting one, and can also refer you to an abortion clinic or pregnancy advisory if God forbid you are pregnant. However, I really just recommend getting a test from the local pharmacists soon (it will only be accurate in a few weeks however) - trust me your parents won't find out and the person behind the counter will not care in the slightest. If you're really concerned, you could get a friend to get it for you.
For the time being, try to calm down. I know how all-consuming and terrifying it can be to be young and worried about pregnancy. Trust me I know - when I was 15 I had unprotected sex and had to get the morning after pill. It was so so scary for me but I felt so much better after visiting the clinic. With stuff like this, you need to take action. Go to the clinic, go to your GP, talk to someone because it will not just go away and if you do choose to opt for an abortion, it is easier, quicker, less traumatic and safer when it's under 8 weeks.