Hello there S,
Welcome to the forums. I am very pleased to meet you because you've taken a really big step in booking an appointment, and I understand you are feeling quite confused about whether you've done the right thing or what to even expect.
To answer your first question, you have absolutely done the right thing by seeing your normal GP. GP's are also trained to assist and are often the best way to help you understand your options. While there are further professionals who specialise in mental health, it may not be necessary to go to them straight away and your GP can help you better understand the pros and cons of doing so. Also, GPs are able to provide a 'mental health care plan' which provides a number of subsidised sessions with a specialist.
In terms of how to prepare and what to expect, a lot of that will come down to your particular doctor but more than likely, they'll chat to you about how you are feeling, your history with mental health and they may give you a little questionnaire to fill out which gives an indication of your depression, stress and anxiety levels. They may prescribe antidepressants as well, but that is something to talk to your GP about.
It is not supposed to be a confronting experience, but I understand your concern and a lot of people also mention feeling a little uncomfortable because it's difficult to talk about our issues. So I think your idea about writing some notes will be really helpful because, even if you can't verbalise them in person, you can show them and have them read it.
Feel free to let us know how it goes. I'd love to hear back from you.