First download a Raspbian RAW image from here: http://www.raspberrypi.org/downloads
Plug-in your SD card into your PC. Then use df -h to see the name of the disk device of it:
$ df -h S.ficheros Tamaño Usado Disp Uso% Montado en /dev/sda1 55G 12G 40G 23% / udev 994M 4,0K 994M 1% /dev tmpfs 402M 804K 401M 1% /run none 5,0M 0 5,0M 0% /run/lock none 1003M 768K 1002M 1% /run/shm none 100M 32K 100M 1% /run/user /dev/mmcblk0p1 3,8G 4,0K 3,8G 1% /media/SDCARD
In my case the SD Card is mounted as /dev/mmcblk0p1. Once you know this unmount it:
$ umount /dev/mmcblk0p1
Now it’s time to copy the Raspbian image in the SD Card using dd but we don’t want to write it to the partition but to the whole device, so we have to remove the last p1 from the mounting point:
$ sudo dd bs=4M if=2014-01-07-wheezy-raspbian.img of=/dev/mmcblk0
Finally flush all the data just in case:
$ sudo sync
Ref: http://elinux.org/RPi_Easy_SD_Card_Setup#Using_the_Linux_command_line