If you are going to a gender therapist I would let him/her ask the questions, beginning with what will likely be among the first: "Why are you here today?". You supply the answers. If they wish for you tell a condensed version of your story as to what makes you think you are transsexual, give them a brief accounting and history as to what makes you suspect you are transsexual. There will be further questions as the therapist needs to know you better. The first visit will be for an evaluation and after that it will probably be weekly or monthly visits to determine your state on mind and and how you are progressing.
At the first meeting you should ask the therapist to assure you that all that is said is kept in the strictest confidence. When you have established that, please, don't hold anything back. No therapist can assist you if they don't know what's going on inside of you.
Cindy