Do you know exactly what you're going to tell them? That's probably the first step - have a plan for the points you want to get across. That you're transgender? Transsexual? Going to transition? Or just communicate your feelings about it all? Etc.
Anticipate questions. They'll probably ask if this means you're gay, how long you've known, why they never noticed anything, etc. All common questions it seems.
They'll probably want to know what this MEANS to them and ask what are you're going to DO about it?
~Kate~