You can download and install R for free from the following webpage: http://www.cran.r-project.org/
At the top of the page, select your operating system (Linux, Windows, or Mac) and then follow the instructions.
For most Windows users, you will select "install R for the first time" and then select "Download R 2.15.2 for Windows" at the top of the page.
For most Mac users, you will want to download R-2.15.2.pkg (unless your operating system is earlier than Mac OS X 10.5 Leopard).
After you have installed R, please complete the "Testing R Installation" assignment.