Preparing, formatting and writing an SD card for your Raspberry Pi is now less of a struggle thanks to very clever software across all platforms. You can also easily obtain readymade SD Cards from online stores and suppliers if you don’t feel like making your own. Operating Systems for Raspberry Pi are mainly based on Linux Debian and comes in various mutations according to what features you would like. The Software is distributed freely on the Raspberry Pi Foundations’s website, either as ‘standalone images’ or New Out Of Box Software package (NOOBS). As a beginner I would highly recommend using NOOBS for ease of setup, you will be guided through the installation process from beginning to boot up. The most popular and stable standalone distribution at the moment is Raspbian Wheezy (Debian) – this will give you a bit more control over installation , you can download all the software here. We will go through platform specific installallation on the next pages; installing NOOBS is pretty straightforward.
Once you have downloaded and unzipped the NOOBS folder you need to format the SD card using SD Formatter, available for both Mac and PC. Having installed SD Formatter, insert your SD card and open up SD Formatter from Start Menu (PC) / Applications Folder (Mac): You should see this (Mac)
Ensure the correct SD Card is selected and Choose Quick Format unless you have an old SD Card that has been rewritten a lot.
Locate your NOOBS folder and copy ALL of its contents to the SD Card but NOT the folder itself. The card is now ready to be inserted in to your Raspberry Pi and booted up.
Full Guide here