Greetings
First format your SD card in FAT32 and download the file that say LINUX IMAGE FOR SD for Intel Galileo from here: https://communities.intel.com/docs/DOC-22226. Then extract the file with 7zip if you are using Windows you can download it from here http://www.7-zip.org/ or in Linux type in the terminal sudo apt-get install p7zip-full. Extract the file and then put all the content inside the SD card. Put the SD in the socket and power the board and you are ready. You can check if it worked by uploading the Blink sketch, then disconnect the board and a few moments the blink sketch should start by itself. Then if you want to enter into Linux I recommend you to visit this post https://communities.intel.com/thread/46335 it has a tutorial on how to enter Linux console of the Galileo.
Regards
JPMontero_Intel