My personal opinion is that a good therapist is going to let you bring up the things you want to talk about at your own pace, so you'll start to talk about the gender stuff when you decide you want to bring it up. Those who have had experience with trans patients have probably seen them come in at all ends of the spectrum, some just seeking a letter for HRT, some wanting truly to figure out if they have some form of GID, so you don't have to have the mindset that you're transitioning tomorrow right at the start. It's also not an easy subject to talk about, especially if it is something that you haven't shared with anyone before so I can't imagine that any therapist would expect you to be completely blasé talking about it with a stranger. Like the others have said, it's a relationship that you and your therapist are going to develop, and that takes time too. In the end though just be yourself and go at whatever pace is comfortable. Someone with experience should be able to pick up on that, let you do the talking, and know what to ask when it's pertinent. Remember that they are there to help you figure things out, and that they are working for you, you have every right to dictate your expectations.