The best way to back up a WordPress site is by using a dedicated backup plugin or service such as BlogVault. Besides backing up your website, such a service will keep old copies of your site so that you can restore to any backup point up to 1 year ago. If you want to perform a backup manually, you'll need to create an archive will all of the existing files in the WordPress folder and a full export of the database associated with the current website. For large sites, manual backups can get complicated very quickly and are not very reliable.