This online book assumes that you are familiar with another programming language such as Basic, Java, or C. However, this is not a prerequisite, and you shouldn't have particular problems if J is your first computer language (in fact, congratulations!).
It is also assumed that you are familiar with running Windows applications, in particular MDI (multiple-document interface) applications such as Word and Excel.
Most things can be done in J much as they are done in other languages, and in several areas a topic is introduced just as it would be introduced in other languages. If you are familiar with other languages this makes it easier to follow how it works in J. In some cases there is a much better J way to solve a problem, and that is also covered.