Do you know if your parents are accepting of transgender people? If they aren't, it might not be a good idea to come out just yet, or at least make sure you know you have a safety net of other people that can support you. You could try writing them a letter, which is nice because it gives you a chance to think about what you want to say, and review it first.
Exactlywhat ACM said. At 13, I wouldn't recommend coming out to them unless you are certain they will be supportive, and even then, others first. Have an outside support structure in place first, just in case. at 13, you still have several years you can expect to NEED support typically provided by parents (food, shelter, clothing...)
