Best of luck for the appointment tomorrow. It can be really difficult to talk about it but it is worth it to start the path to getting better as soon as possible. As someone already mentioned GPs see a lot of people for depression and so yours should be understanding and able to offer some help. A few extra things --
I'm unsure if your doctor will do this, but at the doctor's surgery I go to, they give questionnaires for anyone who comes in with mental health problems to fill out. They cover a few of the main symptoms of depression and anxiety. They are very quick and easy to fill out if you do get one (you just have to circle to what extent each symptom applies to you.)
You didn't mention your age in the OP...what will happen will probably differ depending on whether you're over or under eighteen. If you're eighteen or over and the doctor diagnoses you with depression it is fairly likely (though not certain) that they'll offer you an antidepressant. Usually they start with one of the SSRI antidepressants (e.g. fluoxetine, citalopram, sertraline, etc.) which help a lot of people but of course nobody is going to force you to take medication if you would prefer to try alternative methods first. Hopefully they should also refer you to some kind of counselling/therapy (often CBT) or at least give some other advice or options. But waiting lists for therapy do unfortunately tend to be very long.
If you're under 18, the doctor may not be able to prescribe antidepressants to you so most likely you'll get referred to CAMHS (Child and Adolescent Mental Health Service) where they'll assess you and decide what approach is best; whether therapy/counselling of some sort or medication, in which case you'd see a psychiatrist who can diagnose you and prescribe it.
I've been in similar situations both when I was younger and at age 18. The nervousness is totally understandable but I'm glad you're asking for help anyway. The advice some of the above posters gave is great too, and I'd definitely second writing things down if only as something to fall back on.