I just download a .img file, and formatted my SD card. Now, what should I do in order to install the distribution on the card for raspberry boot?
Simply use WinDiskImager. Just select your file and drive and hit write.
You don't tell us which .img you downloaded or what you've already tried.
You can read here for how to use Win32DiskImager: http://elinux.org/RPi_Easy_SD_Card_Setup#Flashing_the_SD_Card_using_Windows
Or you could try NOOBS instead, that's another way to do it and very very easy. Just unpack the zip to the sd card. See here.