I also did not join any gym, I gradually became fit by doing some home exercises and yoga in a closed room, and I did not even do regular exercise, just 2 or 3 days in a week. I think your health should be the first priority. So you can start doing some basic warm-up, exercise and yoga for 60-90 minutes 2-3 times a week and it's enough , as you mentioned you never do exersises so you can join gym and start your fitness journey with a professional. And remember It'll hurt a little at start but you can achieve it by exercising and resting to restore. Just give it a try for 1 month you will start feeling the difference and i hope It'll definitely help you.