Hello. Not sure if you've managed to speak to your boyfriend or your mum yet, I'm hoping that you have so you're not still going through this all by yourself. If you haven't told either of them yet, choosing somewhere quiet when you're both alone to break the news is best, I think.
Maybe you could tell your boyfriend first then both tell your mum together if you're afraid of telling her on your own. Your mum will hopefully help and support you, she's obviously been pregnant before and will know exactly what you're going through. She may be really shocked at first or even angry but I'm sure she wouldn't want to think of you having to go through being pregnant without her by your side, she's your mum after all. Talking about it should definitely help, so if you still can't face letting your boyfriend or mum know yet, do try and talk to someone else that you trust, like a close friend or a teacher at school. Good luck.