christov is correct. Be yourself because those guys will see through any attempt to hoodwink them. They are there to help you and you are there to be helped so just relax.
If you find that you are not comfortable with the therapist/psychiatrist then seek a different one if necessary especially if you pay the bills.
Nero was also correct in that it helps if you present as typical of the gender that you believe yourself to be though I suggest you do not go over board about things as clothing, hairstyle or anything like that, you need to remain comfortable.
Finally, enjoy it because it is the start of a very interesting journey for you and hopefully a satisfactory resolution .