Hi Megan,
Well I'm 34 so I don't know if my advice is considered valid anymore, but I'll give it to you since you asked and I have some experience with the matter.
I know how it feels to be shunned and thrown away by your family. It's peculiar, really, for people whose opinions I didn't really care for before they became very important when I started transitioning. I've had the most success with my mother, because I worked her into things, slowly. I spent months getting to know her on a deeper level than I ever have before. You would think that we would know the people from whence we sprung pretty well, but no.
There is the very real chance you will lose everyone. It happens to people here all the time, that's kind of why we need to stick together. You can try to avoid it by gradually moving into the role. You said you are 60% sure, so what happens if you turn up one day in a dress talking about genital surgery? Well, people will be shocked and surprised and you probably won't like their reaction. Take it slowly. Get to know your mother some more. Make some tea and talk to her, and do it regularly.
Talk about her, not you. When you talk about your history together, use "I statements" like I feel, I remember...when you use "you" it changes the tone of the conversation and makes people defensive. I would suggest doing this a lot. Nothing endears you to people like talking about and showing an interest in them. As you work through this process you will find the conversations probably will balance to include you more often. I heard a beautiful story the other night about a girl who came out to her mother over ice cream, no drama, no big scene, just basically "mom I'm a girl" and it went pretty darn well considering. Baby steps.
Lastly, try to remember that you have had all the time you need to think about this, they probably haven't. Your reaction to their reaction is important too. There's a good chance you will lose everyone no matter what you do, but really reinforcing the relationships first can help a lot. It will make you appear to be making a lot of progress in their eyes, and hopefully the first thing they will associate with you being trans is not showing up in a dress with a beard, but with being a more emotional and engaging person to be around.
Oh, and ignore the nonsensical hate speech and do listen to the older people. I agree that a sense of urgency is in order at your age, but that's where it stops. There are several great role models here, all of them over 35. One in particular is my heroine, when I'm having a hard time keeping at it I look at her old posts as she documented going through the very same things I am; and ending up with a fabulous life as a blended, normal, well adjusted woman.
