Which part of rsync do you want to turn on? If it is the server part, then you just need to run
Code:
${PATH_TO_RSYNC}/rsync --daemon
plus, you want to make sure you have the /etc/rsyncd.conf file setup properly for your environment, it will be different for everyone.
If you are talking about the client, then there isn't any turning on, so to speak. You execute rsync with certain parameters to start syncing data, again, this will be different for everyone.
There are plenty of examples of how to run it and the Rsync link given by CrazyLane is where I learned how to use it.