I was a bit nervous, which was made a lot better by the fact that I had a friend with me. First session is usually just talking about things like "why did you decide to come here", what do you struggle with (broadly), what do you study/what's your job, are you in a relationship... you're getting to know each other. That also means you can ask the therapist questions. How long does he think it will take, how does he usually work, what are the policies on outside contact, when would your sessions be and so on.

I took about 6 months until I talked about actual problems, so being reserved should not be a huge issue. Also, I think most people contradict themselves sometimes. At least I do. I don't think it matters a whole lot. If anything, the therapist then knows that you are not too sure about some things.

Try reminding yourself that the people you will see are professionals. It's very normal for someone to be nervous in therapy. Not only on the first session. They also won't judge you. If you say "I'm nervous so maybe I say something wrong" they will probably understand and even give you some good ways of thinking about it.