There is not a single way to learn anything. I would say get the lesson and spend some time on your own on a 20ft sailboat too.
My first sailboat was a styrofoam disposable paper cup kind of a thing called a Sunflower. After a few days of sailing it, I knew I liked sailing, so the next boat was a Cal 2-27 with hurricane damage that I had to fix and later live aboard. I had some guidance from an old salt for the fixing part, but never took a single class for sailing. One could say that shows, and if I was into racing I would probably crew for an experienced team first, but seamanship is mostly knowledge, experience and intuition, which you can get from reading and messing about on boats.