Installing an operating system
The raspberry Pi can be purchaed either as just a bare circuit board or bundled with related items. The most common item included in a bundle is an SD card (or micro SD card and SD adaptor) preloaded with an operating system.
If you have pruchased just a Raspberry Pi, you will need an SD card or micro SD card and SD adaptor. 4GB should be considered as a miniumum, but I recommnd 8GB or larger. This is because a typical OS like Raspbian wheezy will leave just under 1GB of free space on a 4GB card. Once you have added some other application there may not be enough room for your own files. Even if there is a small amount of space free, this may be consumed by the operating system and application log files. If the memory card gets full the operating may fail to boot fully.
There are several operating systems available for free on the official Raspberry Pi website.
Raspberry Pi OS downloads
I am using Raspbian Stretch on this web server - a variant Debian Linux.
The operating system is downloaded as an img file. This img file must be copied to an SD or micro SD card. There are instructions on how to do this on the Raspberry Pi website, but in my case I used my Asus 900 netbook running lubuntu and the dd command. If you are using dd be sure which device you are copying to as there will be no "Are you sure? " confirmations. In my case my SD card was /dev/sdc and my USB memory stick with the .img file on was /dev/sdd1
First I needed to mount the USB drive using:
sudo mount /dev/sdd1 /mnt
then I copied the img file to the SD card with:
sudo dd if=/mnt/XXX.img of=/dev/sdc
where XXX.img is the operating system image file.
This is not just a simple file copy - it creates a bootable device based on the contents of the img file.