Hi Carolyn,
The first thing I would recommend is finding a transgender-knowledgable therapist. If you are like many transgendered persons (including myself), you have had this gender issue bouncing around inside your head for years - with no one to confide in. Even if you are lucky enough to have someone you can trust enough to talk to, a professional therapist can help you decide what rational steps you can take, and how to deal with it. Transitioning for most people is very difficult. It involves the likelihood exposing yourself to ridicule, rejection, and pain. Losing friends. Losing family. Losing your job. You have to balance that potential loss against what you feel inside, and what will really make you happy, beyond "I want to be a girl".
Before you discuss this with your parents, you have to know yourself. I know you feel you are female, but what does that mean to you? Just as there is a spectrum of human sexuality, gender is not necessarily binary. Do you feel more female than male, but like some parts of being male? If given a chance, would you live as a woman 24/7, or do you just like dressing up once in a while? Will nothing less than sexual reassignment surgery satisfy you? A therapist can help you with these questions, and also provide a safe, protected place when you decide you need to tell your parents about your plan to transition.
It is often said, "the difference between a goal and a dream is having a plan". If you are serious about transition, and want to make it more than a dream, then you need to identify what it is you really want - in specific, achievable terms - and outline plan to make them happen. That includes financial plans, because transition is not cheap.
I hope this comes across as supportive, because I am. I am much older than you (58) and can only wish I had the courage to do 30 years ago what you are contemplating doing now. But it was a much different era 30 years ago, and in all honestly, it could have turned out badly. It doesn't matter. Each of us decides when we are ready. It sounds like you may be ready now. But be kind to yourself! For instance, I have given myself permission to fail at this. I am being very careful and deliberate each step of the way, and anticipating the consequences. If it gets to be too much, I may have to give up. In fact, I have read several stories on this site of people who did try to transition early in their lives, and had to stop for whatever reason, then found the courage and opportunity to try again. You are still young, and have many options. Listen your heart, and listen to your life.
With kindness,
Terri