IOS & objective C development - how to make the login page appear to the user?

I am in my first days of IOS app development, I am trying to build an authentication system for an already existent iOS App using Objective C.

The app's rootviewcontroller is a tabsview followed by navigationControllers.

What i've done so far:

1- creating the loginviewController class & designing it's UI in the storyboard

2- the same thing for the "registration" & "recover my password" classes

3- linking the root viewcontroller with the login page with a segue of type modal.

4- linking the login page with the registration & recover my password pages with segues of type push.

Now i don't know the steps that i should follow to make the login page appear to the user when he first enters the app & eventually store his state so he can access the app later without having to enter his credentials every time (unless he logs out).

Any help is greatly appreciated, thank you

I am available for any clarifications or eventually some screenshots/source code if needed.