A friend of mine and I are planning a game together to work on in our free time. It's not an extensive game, but it's not a simple one either.

He's working on the story behind the game while I'm working on the graphics and code.

I don't really know where to start with the game. We know what the basic type of game it's going to be and how it would be played, but I'm having a hard time of actually knowing where to begin.

I have Xcode open but I don't really even know what I should be designing first.

What is some advice for this writer's block? Where is a good place to start with a game? Should I design all the graphics and layout before even touching Xcode? Should I program the things I know I'll have difficulty with first before getting to the easy stuff?