Developing iphone game

Hello, i am a very new beginner of apple developers community, i am computer science graduate (2 months ago), very interested in games, specially WoW, Diablo, Dota etc, and interested to start developing iOS games, i just bought macbook air 13 yesterday, been learning and reading a bit before i get it about objective c and Xcode work.

I wonder how hard/long it would take to develop a simple yet exciting game, something like free running games, or simpler games. Also where is a good place to start? what strategy should i follow? watching tutorials? follow a sample project? what about graphics work, pointing that i don't have experience in it.

If you want to add any advice or recommendations, i would be more than happy.

Thank you for your time.