What you need to know to learn the code.
I recommend that you start at Intro.txt and work your way through the lessons in sequence. When a lesson file is opened the supplementary lesson window will be displayed where you can read instructions on how to work through the lesson. When you click on Stop after playing some code a secondary message will often be displayed to give instructions on what to do next.
Technical information on lesson files.
The lessons are a set of files in a subdirectory called lessons. They are text files containing special instructions that set up NuCode for each lesson. They will usually also contain text to be played as code. The introduction is called Intro.txt while the rest of the lessons are called LessonNN.txt where NN is a number.