There is a tremendous amount of information about Arduino on the web.
The “Introductory” section of this Manuals and Curriculum page on Arduino website has an excellent and short set of links to books, tutorials, and manuals.
The definitive source of information is the Arduino home.
Instructions for downloading and installing the Arduino software on your computer (free and safe) is here.
A small number of inexpensive components are all that is required to implement most of the tutorials and examples.
For a deeper understanding of basic electronics
Arduino Duemilanove schematic is here.