For some time is worrying.... I would have expected fed you would have known all the way through your young life if you were the wrong sex, but talk it over with your family, and if they are comfortable with it, try like living as a girl when you are at home. When you are really sure, then ask you GP for help and a referral to the appropriate service in your area. It may be enough to have a female persona you can switch to for a while. It may also be worth looking for young trans groups on -line and exchange stories.