I built the backend server with Rails (I knew this already), but the rest was native iOS, so all Xcode. As far as how to architect it, I'm not sure I'd be much help. I basically jotted down what the app would look like and just started with individual features. I did have some help and advice from two iOS devs when it came to questions like, "I want it to do X, what is that called in the iOS world?", so I was better able to search for answers.
Basically, I'd just start with a feature of your app and try to build it. Most likely, someone smarter than both of us has done it and written about it (or at least something similar enough). Once you start writing code, you'll quickly get a feel for how things work and have a better sense of what the overall app should look like. Not sure if that helps any but good luck.