Even if you've done C++ before and want to target mobile, I'd still recommend starting with a forgiving language such as Python. Python is simple so you can be immediately productive. It has lots of documentation, it's easy to search the web for examples and even if you won't use it for mobile it will remain a valuable skill.
Mastering a simple language can also be like the first 50% of your goal. While Python is not exactly the same as something like Objective-C, it has many of the same concepts (classes, containers, C-like language features, etc.). You will quickly become comfortable with relatively complex programming tasks, and you won't be bogged down by tangential stuff like compiler errors, Interface Builder, simulators, or who knows what else.
Don't be afraid to start building things that you may not need yourself. For example, you may have no reason to build an interactive shell that acts like a calculator, but it's a tiny project that would slowly introduce you to a few Python library functions and language constructs. It's also pretty obvious if a calculator is behaving correctly. When that works, you should ask yourself how you might extend it; for example, if you wanted the calculator to write its results to a file like an adding machine writes to paper, search the library for functions that deal with files and try to learn those too. Do this for awhile until you have a few generations of the thing and you've learned a few diverse things.
Only then should you try to map what you know to a more powerful environment that is more closely aligned with a mobile platform. You will find that once you're very confident in one language, a 2nd language such as Objective-C isn't too bad. It's a lot of different syntax and a whole new standard library, but you will find both the language and the library to have similarities (e.g. you can teach yourself NSDictionary by remembering how Python dicts work, or NSArray by remembering Python list capabilities). Yes, you still won't learn it overnight, but it will be easier than starting from scratch.
And since you have a working prototype in another language, you can then see what it would take to map this to the new environment. For example, pull up a template project for the iPhone, and try to make a very basic calculator app by making buttons perform the actions you used to receive from the command line in the Python version.