If you are using a self-hosted WordPress site, the simplest way to backup your site is to use a WordPress plugin such as WPBackup or W3 Total Cache. These plugins will automatically create backUPS of your site every day, and you can also schedule automated backUPS using an automated backup service such as Crashplan.
If you are using a hosted WordPress site, your host may provide a backup service. If not, you can find a number of third-party backup services that will backup your WordPress site.
Always make a backup of your site before making any changes to it, just in case something goes wrong.
Finally, always remember to keep a backup of your site offline, in case your online backup is lost or corrupted.