You will not be asked to lie on a couch, and nothing has to be decided in the first hour. A first session is mostly your therapist getting a sense of what brought you here — and you getting a sense of whether this is a person you could talk to again.
Expect some practical ground to be covered: how long sessions last, what they cost, how cancellations work, and where confidentiality ends. Good therapists explain that without being asked.
You are allowed to say you do not know why you came. “Something is not right and I cannot name it” is an ordinary place to start, and often the most honest one.