The Raspberry Pi has been available to order for a few months now, so see if you have $ 25 left over, you can get it right away. Equally good is that some improvements have been made, such as in the case of RAM which has been expanded to 512MB in the latest model (raspberry pi model B). It uses Linux operating system, Broadcom BCM2835 700MHz processor ARM1176JZFS FPU and Videocore 4 GPU, has Ethernet socket, HDMI port, USB, RCA video socket and microSD card insertion for additional space. It measures just 85.6 x 53.98 x 17mm, charges quickly via the microUSB port and includes a 3.5mm jack to connect headphones / speakers.
You can order it here: Raspberry Pi store
For your first steps once you get the Raspberry Pi you will need the corresponding operating system (Linux) and there is a pdf on how to get started with the Raspberry Pi as you will first need to format the SD card on your computer.