One of the resources that helped me a lot was the Big Nerd Ranch. I strongly recommend both their books and their bootcamp training sessions. And as Al Pascual says the best way to get up to speed is to jump off the deep end. Build an app.
When I went to the ranch I had already gone through iOS Programming: The Big Nerd Ranch Guide on my own and we were prototyping the Explorer application. So during labs and after hours I was able to engage the instructors with questions and pick their brains about best practices.
One thing I didn't expect was how hard it was to transition to the Mac and Xcode. My initial feeling was that Xcode was a far cry from Visual Studio and the Mac was a black box OS. Of course that couldn't be further from the truth. I just needed some guidance and experience.
Feel free to use the GeoNet community to ask questions. There are lots of valuable resources here.
And check out Swift for sure. If you've programmed in C# you'll find it pretty similar.
