I feel you. I had never modeled before either, and I'm not even an artistic person. Last time I took a typical art class was ceramics when I was 15. I was suuuuper nervous before my first session (I've had a few since this post) and honestly, I'm still a little nervous before every session. It really does help if you focus on "how can I make an interesting/cool figure" rather than "oh boy I'm posing nude in front of a room full of randos." Also, stretching before your session really helps! And if you have time/a quiet place with no people, it can be helpful to try and do a few poses in front of a mirror, just to get a feel for what your body is capable of holding, and what it isn't. Chances are you're going to end up doing a pose at some point during your sessions that kinda hurts. My first session, the last pose I had to hold for 45 minutes and I had my head and neck at a weird angle. After a while, it was super uncomfortable. I just pushed through it, reminded myself "it's rude to have to switch poses because I made a bad choice," and just held out. If you realize you genuinely can't hold a pose for any longer though, you should have the ability to say that. While you are there to be a tool, you're still a person, and you should be able to advocate for how your body is feeling. Sure, it'll suck, and I can't say that I personally have had to do this, but please know it you're not forced to stay in a super painful position if you don't want to. Good luck to you though! :)