Google has quite good documentation on that, just follow the instructions at https://firebase.google.com/docs/admob/ios/quick-start
Under the "Game Developers" navigation item they have something specific for Cocos2d-x and instruct to use AnySDK, but for plain ad showing in a Cocos2d-objc game, there's no need to do that, the simple and straightforward examples in the "get started" section is enough. Furthermore, I found it simpler to install the SDK manually by just copying the AdMob part to my project, I had no need for the full Firebase capabilities, as I just wanted to show ads in my game.
Essentially you just use the example from the bottom of that "get started" page, and put the stuff into the
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
in AppDelegate, instead of a viewDidLoad. Unless you have a specific view controller you'd like to use, of course.
Then you implement the delegates as you see fit, and call something convenient like "showAdBanner" from your scene(s), when you want to show the ad banner.