1. The first step is downloading the OS you can do this by going on google and typing backtrack download it will be the first site on the top. Next you should find out what bit your computer is 32 or 64 bit you can do this by going to computer and clicking system properties. Then you should download the latest version of backtrack that is your bit type.
2. After it downloads you should get a program called UNetbootin, once that is downloaded you should run UNetbootin and select the OS you downloaded, you should have a blank USB to install the OS on so plug it in and let the program install the OS.
Backtrack is used for hacking it is completely legal to use as long as it is being used ethically I will do more lessons on using backtrack in later posts enjoy.