With any kind of electronic components/designs, the manufacturer always provides some kind of specification documentation. Same goes with our Beagle.
BB foundation provides System Reference Manual (SRM from now on), and you can find it at http://beagleboard.org/hardware/design. Just check your board revision (it should be printed on your box), and download the correct one. All instructions in this book are based on the first production release for BeagleBone Black called Rev A5A. At the time of writing this book, the latest revision stands at A6.
You should keep in mind that BeagleBone Black and the original white model don't have the same General-Purpose Input/Output (GPIO) pin layout. In this book all the examples are presented on the black model, so please take care if you are using the white model, and follow the guidelines given in this book!
So back to the SRM; this documentation is very useful while planning and designing your own systems, and usually you end up looking...