An introduction to a book or report usually provides general background about the topic, and the nature and structure of the publication.
Some publications have an introduction that is not numbered. This does not work well if there are figures or tables in the introduction, because it makes numbering of the figures or tables difficult. In this case, number the introduction as Chapter (or Section) 1.
The main divisions of a publication can be called chapters or sections, and are numbered with arabic numbers (Chapter 1, 2, etc). Chapters or sections can be grouped into parts, which take either letters or capital roman numbers (Part A, Part B, etc; or Part I, Part II, etc).