I'm not sure on the privacy and security side of things, but setting it up should be a matter of logging in when you upgraded to iOS8.1 (if you didn't do this on setup, then go in to the iCloud Drive tab in your settings and go from there). It should already be set up as a matter of course on Yosemite, but again if it isn't then go into System Preferences -> iCloud and enable iCloud Drive. It will then prompt you for your iCloud username and password.
On Yosemite it will then show in your Finder on the left, and on iOS it will automatically show any files in iCloud within the app.
Hope that helps you get started.
Alex