Your Raspberry Pi needs a SD Memory Card to work because you need an operating system installed on the SD card to use it. I have been using Transcend’s 8 GB Class 10 SD Memory Card which I bought for less then $8, and it has been working pretty well, and it is fast when I use it on the Pi to install different operating systems like Raspbian, Rasbmc, OpenElec, and other operating systems on the Pi . 8GB of space is enough to install a few operating system, and store some files like MP3 files, and video files on the SD Card.
I recommend a SD Card at least 4GB or bigger in size if you plan on using NOOBs to install more than one OS on your SD card. I read online on forums that a 64GB SD Memory card works on the Raspberry Pi Model B. The smallest recommended SD card size is 2GB for Raspbian Image file. However, if you plan on using a more lightweight operating system image like RISC OS, a 512MB-1GB SD Memory Card would be big enough to install your operating system on the SD card. Some operating systems like OpenElec, and Rasbmc let you install your operating system on a USB drive, but you still need a smaller SD card to boot to your USB card, so if you install your OS like Rasbmc on a USB drive, you could use a smaller SD Card for the Pi.
A SD Card with a Class 10 Speed rating would make starting up your Pi faster, and make opening and saving files faster. A Class 4 or lower SD Card would be slightly slower, and Class 4 cards on cheaper to buy.
You would need to format your SD Card to FAT32 before you use Noobs to install your operating system on the Pi.