The first session can go many ways. Online sessions are not that different from in-person sessions, apart from it running via a computer.
Usually, at the beginning, the therapist asks what are you coming with, what is bothering you, or what you expect from their help. It’s up to you how much you tell them, it’s important to feel safe. Depending on this, the next phase of therapy unfolds. Sometimes the topic is clear from the beginning and the therapy begins to develop in the first hour. Sometimes the client comes in with a relatively complex issue and the therapist requires more time to assess what direction to move in. Some therapists ask about the history of the problem, family background and other important questions. It also depends on the therapeutic framework.
Everyone experiences therapy differently, but often the first session itself will help significantly. It’s up to you how much you say, it’s important to feel comfortable. Therapy is “about people”, it is important that the client and the therapist are sure that they can and want to work together and be suitable match for each other. Part of the first session is an agreement on what you will work on, how often, or for how long. And if you are not convinced that the therapist is suitable for you, take your time and do not force yourself to continue. Alternatively, you can try a different therapist.