Please advise: I would like to start therapy, but I don't know what to say

I have a lot to unload, and I don't know where to begin or what I should and shouldn't tell a therapist or anything like that. I don't want to say a bunch of stuff and immediately get misunderstood for communicating poorly/unexpectedly. I want to make the right impression so that my experience with the therapist doesn't become a shitshow that I can't recover from.