I had my first lesson today.
Apparently the parts of me that are supposed to relax (like my hands) absolutely refuse to do so. I also evidently have approximately zero muscle tone in my left arm. I'm thinking it shouldn't be sore after 10 minutes.
Do you have any tips on how to actually relax and hold something at the same time? Does the arm thing get better?
The "arm thing" does get better.
I used to get regular hand-cramps from my death-grip on my viola. With practice, you'll learn just how little muscle you really need to hold the instrument in place. I'm still using too much tension, as it interferes with proper vibrato, but I'm still just a newbie, myself.
Hang in there. It was just your first lesson, after all.