Educate them of trans* issues, your own feelings, how things will change and how things won't change, what pronouns/terms you prefer. It will be a big adjustment for them most likely, even if they are very supportive. Be patient and self-assertive.
Personally, I'm a fan of written explanations to accompany my in-person explanation, just so they can have a reference, as it can be a lot of new information for some people.
It's very individual to you and your family situation. I recommend lurking the coming out board for a while. It's also a good place to share more personal information about your family set-up and other relevant factors in order to ask more guided questions.
Best of luck, I hope you sick around on Susan's
