One of the absolute best things about the Arduino platform is how easy it is to get started. The software that installs on your computer is completely free and designed specifically for ease of use. The program is called an Integrated Development Environment, or IDE. The fancy name might intimidate you but it runs just like a text editing program.
As with any software install, you may have some peculiar things working on your computer that could hinder a smooth install. I have loaded the Arduino several times on different operating systems and have not had too many troubles. Once or twice I have had to re-download the zip file that contains the Arduino IDE, because some how or other it got messed up during the download process. You may have to install drivers yourself for your Arduino board - this turns out to be fairly easy and is clearly explained.
In order to provide you with the most accurate written information on downloading and installing the Arduino IDE, I will point you to the Arduino "Getting Started" page. They have done such a clear and concise job at describing the process, I see no reason to repeat it here.
This Arduino tutorial was created by Open Source Hardware Group. We are an education company who seek to help people learn about electronics and programming through the ubiquitous Arduino development board.