Beginner resources device design/assembly
An Arduino is a great way to get into the world of embedded devices and systems, which is what I think you are (mostly) referring to. With Arduino, you already have all of the components assembled for you, which is the hardest part, in my opinion. You can then become familiar with the important aspects of embedded devices, such as interrupts, memory-mapped I/O, etc. There are lots of choices, and you can work with a ATMega or ARM core processor, depending on the board you choose. I'd personally recommend beginning with the ATMega, (as comes on the UNO), as you can find lots of information for low-level operations later on. 

Once you feel comfortable using the integrated IDE, you can switch to something more advanced, and program using just the barebones (registers, low-level syntax, etc). There are plenty of tutorials showing how to use eclipse IDE with Arduino. It's important to become familiar with the barebones, as most board-level microcontrollers will require you to interface with them in this manner. Many use C, but depending on the compiler and development environment you choose, you can also use C++, assembly, etc.

Finally, when you are ready, you can then move up to your end goal: creating your own embedded system using just the bare microprocessor. This is difficult, as you'll need to set up a lot of the basics that the Arduino already takes care of, such as the hardware clock and serial interface (most Arduinos actually use another ATMega processor to take care of the serial), but once all is said and done, it's very rewarding.