The simplest way is to go into System Settings and User Accounts and set their account to log in automatically. Then go into Security & Privacy and uncheck "Lock on Sleep" and "Lock after screen shuts down."
As far as using the interface is concerned, they shouldn't have to input a password at all.
To make it even safer for the end user, you can set up their account as a "Standard" (i.e. non-administrator) account, after which you can use the Parental Control settings to limit their access to different programs.
But this is all basic system-admin stuff. Suffice to say, you should set them up with their own "Standard" account and set that account to log in automatically and the screens not to lock. If you, as the system administrator, ever need to get in and do things all you have to do is log out of their account and back in as your own.
This won't allow them to do their own system administration, but I suspect you don't want that anyway.