Creating a backup of your website
You should backup your website on a regular basis, weekly or before and after you make any significant changes is recommended.
Previously, backups were able to be stored on the server of the website. This is no longer available using the WordPress backup facilty. There are dozens of Backup Plugins available, but I prefer to backup a websites files and folders by copying the entire site via FTP to a local computer, then backup your database via the WordPress instructions link below.
Backing up files and folders
Using an FTP client (eg Direct FTP by Coffee Cup), with your FTP settings (in your hosting email), FTP (copy, not move) your website folders, including root directory to your local computer, making sure you keep the same folder structure. In the event of a website failure, these files and folders can be copied back to the server in the same way.
Backing up your datebase
See WordPress instructions here.