My suggestion as I just came out, is to take it in parts.
I haven't mentioned to my mother that I've got pretty strong gay leanings, she can deal with that AFTER she learns to be ok with my transition. It's pretty stressful from the shock point of view just dealing with one surprise at a time. Let your parents have a pretty long time to absorb and come to terms with transitioning first.
Stress a few things to them.
1. You LOVE THEM. You need them there to be part of your family. Always bring it back to how you love them, because your parents need to know that. It also helps to calm the situation when your mom might feel like you're coming off as aggressive. (Try to avoid IM GOING TO DO THIS AND YOU CANT STOP ME, statements. They come off as very aggressive, and can quickly turn a neutral, worried, or just confusing reaction into a highly defensive reaction on your mom's part. If you need to, try to explain it in a way that says this is the only way you can feel whole. It focuses on your needs, but doesn't sound like it's attacking her concerns.)
2. This is not something we choose, and outside sources did not cause this. Childhood traumas do not cause this. It's about the way our brains interact with information.
3. This is not fun. This is not play. This is a serious medical condition that can cause depression, and DOES cause severe anxiety. It can lead to severe mental problems, and even lead some people to considering suicide.
4. There's no cure, and never has there been a documented medical story of a transgendered individual being 'fixed' by any other means than medically transitioning and living the life of the opposite gender as they were born.
5. This is not about who you want to go to bed with. This is about who you want to go to bed AS. So you can be at home within your skin, because right now your body is your prison.
6. There's resources out there to help you, and to help her understand it. And most of all, you're happy to tell her as much information as she's ok with, but are also willing to bring it up less if she's uncomfortable at that moment. Let her know that you want to accommodate the fact that this will be hard on her too. Make sure she knows that you understand this is confusing for her.
I'd suggest writing a letter. Explaining all of this, explaining when you knew you were different, any instances from your childhood/teen years that they might be able to remember that spoke to you as important in this journey so far to deciding this. Explain how you feel.
Ask them if you can find a local counselor/therapist for you to see. Either going alone or together to work on all of this. (Ask for YOU to find them, so you can find someone who has experience dealing with trans/gender issues in youth, in your area.)
Maybe before telling them, find an adult at school like a trusted teacher who you can come out to, and see if they'd be able to help you talk to your mom. Or maybe if they'd be alright being a resource for your mom to call, and talk to. A school counselor can also help with this.
And above all else.
When you write this letter, and are ready to tell your mom, do two things which will help a lot.
1. Read this letter to them, on a day where they wont be going to work anytime soon. Or when they've had a decent day at work. Read it to them face to face. If you cry, or get stifled, or your throat closes up then use the letter like a script. Something where you can read it and keep going no matter how difficult it becomes.
2. Before you read this, ask her to promise to listen to you, and try not to interrupt you because this is a very important conversation, where you need to read this to her. That in and of itself could be the difference between a yelling match, and her listening. If she agrees, then begin reading it, and try to look at her while you read it, and keep eye contact as often as you can throughout the process of reading.
Remember, appeal to her through facts, but also appeal through emotion. Remind her that you're the same person you've always been, and that you love her, and need her to be there for you as she always has been; your mother, who's always protected and taken care of you. Because this is going to be scary, and you need her.
(And this part is hard, but if you feel like your mother has not protected you, or has not taken care of you, try not to mention that, and still remind her that she has protected and taken care of you, because she needs to hear that. Don't let this turn into an argument. Stay as calm as you can, do not yell. Even if she's yelling. Stay stoic, and try to love her through the difficult parts of the conversation. Coming out at first is hard, it's confusing and stressful and worrying on all parts.)
Best of luck Rhydian.