This took a lot of courage and I respect you for sharing this. I'm not sure what I would do in your shoes, but I would probably try to wait before telling anyone that you think might not be supportive. You're young and they'll probably just call it a 'phase' that you'll grow out of...Even if you know differently.
Oh, I misunderstood when you said "I'm now around 12 years old"...If you have anyone in your life that you feel will support you in this, start by telling them first. You'll then have some of the weight off of your shoulders and maybe they can be with you/support you when you tell the rest.
If this is how you feel you need to live your life, then do whatever it takes to be you. Just be prepared for bad reactions :(
if you truly want to tell it, just yolo it, and if some bad things will be thrown on you, fight trough it and it will be mkay at the end. with other situations to, yolo it trough, and if it goes to shit, just fight trough the pain, it will be okay in the end, and if you can't fight trough it it is always a way called suicide, which is a road i do not want you to take. as Shia LaBeouf says JUST DO IT!!!
Gender and sexuality are two different things, so I didn't quite understand what you meant when you said this- Mind explaining? Also, I'm glad that you feel comfortable sharing who you are and I actually would've told you to come out to your family if you wanted to, but @DeathEssence is right. It shouldn't change who you are or what they think of you, so it doesn't matter if you tell them or not.
If your parents well and truly love you, they will be accepting and show forgiveness for the chooses you have made. Sexually doesn't define who you really are on the inside.
