If you've never turned on your BeagleBone Black, there will be a bit of initial setup required. You should follow the most up-to-date official instructions found at http://beagleboard.org/getting-started, but to summarize, here are the steps:
Install the network-over-USB drivers for your PC's operating system.
Plug in the USB cable between your PC and BeagleBone Black.
Open Chrome or Firefox and navigate to
http://192.168.7.2(Internet Explorer is not fully supported and might not work properly).
If all goes well, you should see a message on the web page served up by the BeagleBone indicating that it has successfully connected to the USB network:
If you scroll down a little, you'll see a runnable Bonescript example, as in the following screenshot:
If you press the run button you should see the four LEDs next to the Ethernet connector on your BeagleBone light up for 2 seconds and then return to their normal function of indicating system and network activity. What's happening here...