I have been wanting to do sports for a long time, but I can't get too much stress on my body. So I'm thinking of trying stretching classes. What is the best way to start training?

If you are new in this direction, I recommend you to sign up in stretching studio https://flexderek.com/en/stretchingonline/ There you will be taught how to properlydash, and work with your muscles that would not harm yourself. I also like stretching, it is much better than heavy physical sports, and stretching tightens the body.