Here are some suggestion to start learning about Nice.
The tutorial give a short introduction to the most used features of the language.
Learn how to use the compiler (important read, since unlike many languages, Nice programs are compiled by package and not by file). The HelloWorldNoviceExample also has detailed compilation instructions.
The Doc part of this wiki contains additional material, including:
The User's manual has a more comprehensive coverage of most aspects of the language. This is the reference that defines the language. Use it when you need detailed information. It also documents some advanced features not covered elsewhere.