Sometimes parents don't understand right away, but eventually come to understand. I was so afraid to tell my mom when I realized I felt this way that it took me until I was 21 to tell her. She spent years cracking jokes at my expense about it, urging me to pretend it wasn't true around my extended family, and asking me if I would change myself to be more like a man if I could. However, year after year, she saw that this was who I was, and that not transitioning was hurting me. Eventually, she started to ask me softer questions about it, and even urged me to transition. These days, she reads all the time about transgender lives to educate herself about me, and sends me positive news articles about being transgender. She's infuriated that there are people who treat transgender people the way she used to treat me.
It can take time for a family member to understand that this is who you really are - sometimes, it can take many years. However, the sooner you start telling them that this is who they are (and the firmer you are about it), the sooner they can start adjusting to it. At first, they may not believe you, but you are who you are - Neither you nor they can change that, and over time, they will probably realize that who they thought you were was just something they had made up. I wish I had told my mom years sooner, because then she would have believed me years sooner. I also wish I had been a lot more direct with my mom about it. I danced around the issue a lot, instead of just coming out and firmly telling her who I was - that made her question whether or not I meant it.
Another thing I have learned is that you never know how people will react to you being transgender until you tell them. I spent years hiding myself from family members, friends, and coworkers. When I finally came out, a lot of the people I thought would hate me for it totally understood. Some of those people had said things around me that made me think they had a real problem with transgender people. Some of the people I thought would never understand told me that they had known for years.
As others have said, these years are a crucial time for you. Going through male puberty when you are a girl is horrible, and you can't take back a lot of the stuff it does to you. Maybe if you tell your family now, they won't understand or believe you, but maybe they will by the time you're 16. If you wait until you're 16 to tell them, it may still take years after that for them to really believe you. I'd suggest involving them as soon as you can. Be up front about who you are. If they react badly, then just keep on showing them who you are. It might hurt, it might be really hard, but it's very hard to deny something forever when you see it every day.
