Firstly go to www.raspberrypi.org/downloads and download the latest copy of raspbian, right click the zip file and choose to extract, this will save with a .img file name.
Next place the card in the USB reader and plug in. You should already have downloaded sdformatter a sd card formatting tool from the people who decide the specs and standards of sd cards. Open it choose options, choose full and format.
Once done close the program and open win32 disk imager a popular program for putting images onto USB and sd cards.
Once done close the program, take out the sd card and place into the pi, the hardest bit is done.