The first part of your plan should consider all of the things related to the live operation of your website. This covers your hardware, operating system, network connectivity and all of the files and scripts that make up your website. If you own and operate your own web servers, you have to develop in depth plans for various types of hardware failures. But even if you have a hosting package, you still have to plan. Even though hosting companies offer good backups and will move you to another server should yours suffer a hardware failure, your jobs not done yet.
Even with the best support out there from your host, there will still be circumstances to plan for with regards to your hosting. What if your provider's process fails? What if your hosting company goes bankrupt and the servers are unexpectedly turned off? Do you have a plan for this eventuality? You'll have to be sure you've got backups of everything you'll need in case you have to change hosts in a hurry. No matter how good your host is, protect yourself in order to avoid putting your website's future in someone else's hands.