No PC Needed. How to Install Raspberry Pi OS Over the Internet:

As any experienced Raspberry Pi user can tell you, the popular single-board computer does not come with an OS preloaded. Unless you buy a third-party kit / card that comes with Raspberry Pi OS on it, you must download the operating system on a PC or Mac and write it to a microSD card or USB storage device for the Pi. We have articles on: how to set up a Raspberry Pi: and how to: create a headless Raspberry Pi that show you how to do this.

But what if you don’t have access to a computer (or don’t have a card reader)? A new version of the Raspberry Pi bootloader, which is now available in beta, allows you to download and write the OS onto a microSD card, using the BIOS and an Ethernet connection.