I will warn you. If you are attempting to make a very long string and have it be a self colliding object, you will need a very strong computer. I would start with a "noodle." By this I mean just one curve straight curve and then slowly add more and more vertices.
To start setting up your scene just add a plane and scale it up 10. This is just a preference for the size, but you should have something that it can fall on. Next, you will need the "spool" but you could easily just use a cylinder to test how your computer runs it. Next is adding your actual string. Add a curve path and put it above so that it can fall on your cylinder. I would subdivide this curve at least twice.
Make the plane and cylinder to collision in the physics body editor. Set the curve to "soft body" and uncheck "Goal." You might want to increase the thickness of the curve, go to the "Object data properties" panel and under the geometry tab, increase the bevel depth to something like".01" or ".02". You can play around and see what you like
If all of this has worked for you so far, you can start to increase the complexity of your scene and the complexity of your path. Subdivide stretch and shape your string. And that should be enough to get you going in the right direction.