First, install it, using apt, yum, pacman, etc.
Imagine we want to back up files from Directory1 to Directory2, and both are on the same hard drive (this would work the same if the directories are on two different drives). There are several different ways to do this, depending on what kind of backups (i.e., options you want to give rsync) you want to configure. The most general, will be this:
$ rsync -av --delete /Directory1/ /Directory2/