How to start

You only mention Objective-C, but have you also considered instead learning Swift? In the future it will likely become the dominant language of iOS development, and has a gentler learning curve.

If you want to get a job doing iOS development, however, Objective-C will likely be a prerequisite for some time, so you might find it difficult to find work if you can only work in Swift. That said, which ever of the two you learn, transitioning later to the other will be a very small task in comparison to learning whichever one you choose to learn from scratch.

There's a ton of different paths that you can take - Learn the language as you develop and app, learn the language and then develop an app, learn Objective-C straight off, learn C followed by Objective-C etc.

I can only comment on what worked for me, although I'm sure that others will recommend entirely different paths what worked for them too.

I tried a few books that try to teach you the language as you develop the app, but I always felt that I would end up being able to copy and build the examples in the book, but never really understood the code well enough to be able to build my own things. Many of the 'learn iOS development in 24hrs' type books are like this. They throw you straight in to the visual interface building tools, so you can make something that looks like an iOS app and feel good about the progress you've made, but you never get a good grasp of the code.

In the end I got a book called 'Learn C on the mac' by Dave Mark and worked my way through that. The pace of the book is quite relaxed, and the explanations are thorough; I highly recommend it.

After that I worked through 'Programming In Objective-C' by Stephen Kochan - it's pretty dry, but it explains the language well.

I just used some online resources after that to learn the Cocoa frameworks and start build apps.

All in all, I'm glad I took the hard road of just concentrating on learning the language first. I see many other developers who don't have as solid grasp of underlying programming principles as I do. I'm not saying it's the only, or even the best way to go about it, but that's how I got there. I did get a full time developer job in the end too!