dougwong55 has given you some really good links. I particularly enjoyed a book that was written by Jeremy Blum called Exploring Arduino. Besides the link to Jeremy's web page that Doug has already given you here is a link to the web page of the book.
you have a good set of starting point for the Arduino understanding. I suggest also some posts I have published in the recent past on Electroschematics, with documentation, sources, schemes etc using the I2C (two wire protocol) to expand the output pins with a shift register, control a display etc.
