First, you need a Raspberry Pi 2B computer. You can purchase it anywhere. It’s home is here: https://www.raspberrypi.org/
Remember to purchase a micro USB cable with a power charger which is used as the Pi’s power supply, also remember to purchase an HDMI cable.
The HDMI cable is used to display the Pi’s video output on an HDTV monitor (1080p). I’ve only used the HDMI display once to find the IP address of the Pi, after that I typically use the terminal/ssh to connect and work with the Pi.
In case they don’t come with the USB 2.0 external disks (they should), also remember to buy 4 USB 2.0 cables.
Second, you need 4 USB 2.0 external hard disks. The Pi has 4 USB 2.0 ports so up to 4 disks can be connected directly. You can use more disks but you’ll have to use a USB hub. Using a hub to use more than 4 USB disks will degrade performance as a single USB 2.0 connection is shared amongst many disks.
Third, you’ll need a micro SD card, 8GB will do. This will be the Pi’s Linux OS hard disk.
Fourth, you’ll need to download the OS (it’s called Raspbian and is actually Linux in the Debian distribution form. Get it? Rasp(berry) and (De)bian .
You can download it from here: https://www.raspberrypi.org/downloads/raspbian/
Next we’ll write the image to the SD Card …