Would recommend considering this as well. GPs come in all flavours; whilst many are great, others may do the minimum to get you out of the room - I’ve had more than one ask if I’d completed the PHQ-9 recently. When I hadn’t, their response was to give me a copy to complete, telling me to book another appointment once I’d done so. I usually didn’t bother going back.
In terms of what to say, hopefully your GP will prompt you with questions about the symptoms you have, if you explain you’re depressed, but you might want to think about what impact your depression is having on you and how it manifests- are you struggling for motivation and to get out of bed, or having panic attacks and not sleeping from worry? Is there an impact on your work/study/etc?
Mostly try not to stress the appointment itself (easy to say!)- it will hopefully go well and provide you with some reassurance, or an idea of possible next steps, but if it doesn’t go as you hope, try not to feel beaten up about it as these things happen- try to use it as a learning exercise- think about what didn’t go well and what you’d do differently, then try again, possibly with a different GP if your practice has more than one. Good luck!