I have just started counselling: had my second session this morning. Well done on booking a counsellor. I know it can take a while to sort it out.
Before my first session last week, I wrote down everything I felt was important, in case I got there and cried too much to talk. I wrote down the reasons that had made me look for counselling; a summary of past problems; and what I wanted to change.
I spent several days editing what I'd written, really getting it to the point. It really helped to focus my head. And when it came to the first session, I found I didn't cry or need what I'd written: I felt very level-headed and could talk easily.
In the first session, my counsellor told me her fees, confidentiality policy, etc, then asked me about my current circumstances/family/work etc. And we talked about my upbringing/family background. After the first session, she had a general picture of things that have affected me and what I hope to change, and had worked out where the main problems lie. Today in session 2, we talked more about the crappy bits of my childhood.
I've come away both times with things to think about. The counsellor is very astute.
I hope it goes well for you on Tuesday. I really recommend writing it all down tonight, then sleeping on it, and re-reading/writing it before you go.