I think the definition of the embedded systems is changing. First, it was a system which has a dedicated job. However, chip prices go down dramatically and programming becomes easier so it is not really clear a system is embedded or not. I think there are two paths (there are more actually). One is using embedded Linux, you can use beagle bone black or raspberry pi. The other one is bare-metal programming which requires hardware knowledge.
If you want to use embedded Linux, it is pretty close to computer programming. You just need to know some basics of hardware and write your code. It is mainly programming. I am not the embedded Linux user so I can't advise on this option.
If you prefer to bare-metal, you need to learn the details of the underlying hardware. Some cases, you need to deal with registers but this will teach you how the things are working. You also need to know basics of computer architecture because you are controlling a tiny computer and understanding how it works makes things very easy. If you want to be an expert, I don't suggest using Arduino because it use a king of hardware abstraction and you will work more on software. I suggest using TI MSP, it is harder than Arduino but you will know how the things are working. TI is also good for providing the resources, there are many official documents you can find to learn each peripheral.
You don't need to be expert before you dive into embedded systems. If you know the basics, you can start and in a time you will become an expert.
