First off you call and ask for an appointment.
It is pretty much just that simple. The people who take your calls do not (and cannot) ask about what you are seeking a therapist for.
Since you have not been there before you will be asked to fill out a questionnaire when you first arrive. Then you will go into your first session where you tell them what your issues are. This may be covered to some extent in the questionnaire (are you depressed, are you suicidal, have you attempted suicide, are you on street drugs).
If you know the name of the place, you might try looking them up on the web to see if they have therapists that deal in gender issues. If not you *could* ask when you call but that is up to you.
If possible you want a therapist that has gender issue training and preferably one that follows WPATH guidelines.
-Sandy