The knob in the middle of the machine has two parts if I remember correctly, and is the stitch selector. You have to pull out and the turn, both sections turn and should have little metal finger pointers. The little red lever to the side adjusts the width of your stitch.
Do you have the instruction booklet that comes with the machine? That would be a great help for you as you try to learn your way around this machine.
I learned to sew on my mothers 401A and then when I was pregnant with my first daughter, my mom bought me a used one of my own. I have them both now that mum is gone.