The Arduino Integrated Development Environment (IDE) software is a free, open source program that allows users to easily write programs and upload them directly to any Arduino-based system. It is generally based on a variation of the Processing platform, and the two can often be used in tandem for working with Arduino systems. There are many online repositories of open source Arduino code that can be used as a basis for any program users may want to construct.
Arduino IDE is open source software, anyone can download and install it for free on their personal computer.
Tutorials are available at http://www.arduino.cc/en/Guide/HomePage
The Arduino Forum is a great place to ask questions about making a project work.
The SIK Guide Code (v3.2) is useful for getting started.