I have found a tutorial regarding this that I need to go through. My question for now is should my app provide a login screen for icloud? If not how would my app provide access to a specific icloud user?
You don't provide a login screen for iCloud. iOS takes care of that, when the user configures a new device or when they use Settings.app to configure (or remove) an iCloud account. You have no access to the user's iCloud login information. There's only one iCloud account for the device, and it's either there or it isn't there.
The standard way to find out if iCloud is available to your app is to call [[NSFileManager defaultManager] ubiquityIdentityToken] and see if the result is nil. If it's not nil then iCloud is available. You should also listen for NSUbiquityIdentityDidChangeNotification in case this changes.