how to earn being an apple developer
Are you thinking of being a freelancer / indie developer? Or working for somebody else?  Without that in mind the answer varies from 'it could cost you money' to 'whatever the going rate is'. 

Reasoning: if you intend to publish your own apps on the App Store then you will need a developer certificate from Apple, which has a cost to it. For the money however you get access to some excellent tools for developing and publishing apps in a high market place. 

Now what you can earn very much depends on your strategy. For example if you sell your app for $0.99 then after apple takes their cut you will need to have a lot of downloads to make a living from it.  That being said a couple of downloads at that price will cover your expenses. 

If you offer your apps for download for free then you will need another way to generate money. You could include adverts (not a good idea to my mind) or structure your app to include in app purchases. And then we are back to how many people want the extra functions/content. 

There are a myriad of choices so I'd suggest "going to Bing and googling for it"

Good luck. From experience I'd have to say it's a well worth the effort experience.