I'd have to Google it just as you would. Here a few points.
C - Well grab a copy of K&R
Microcontroller: Get over to SiliconLabs (http://www.silabs.com) and grab a development board for something like their C8051C020 along with the free tools. Or you could buy a Keil compiler. Study their sample code in depth.
Lots to learn here: configuring the processor, i/o ports, interrupts, timers, counters, clock frequency, serial I/O ports, etc. Make an LED blink. Then make it blink when you press a button. Output an 8 bit counter to a bank of eight LEDs. Have the LEDs count up and down in binary. Have the LEDs scan left-right-left. Figure out how to read a potentiometer with the A/D. Use the potentiometer to control an output that runs an RC servo. Get creative. Have fun. Google is your friend.
RTOS. Get over to Amazon and get yourself a copy of "MicroC OS II". Once you are done with that book --and fully understand it-- you'll find yourself flying at a different level.
Forth: It's been a while since I've used Forth. "Thinking Forth" used to be a go-to resource. I would imagine it is still of value. Not sure what Forth compilers are available for free today. Google it.
Go over to Amazon and get yourself a copy of "Threaded Interpretive Languages". Either learn a little assembler or use C and the aforementioned microprocessor board to bootstrap your own Forth on the C8051C020. You might want to find yourself a copy of "Embedded Controller Forth For The 8051"
Lisp: http://www.cs.cmu.edu/~dst/LispBook/
I actually learned Lisp while having to customize AutoCAD. They called their flavor of Lisp "AutoLisp". It was a really neat way to learn it because you were dealing with complex graphic entities from the start and Lisp was excellent at manipulating them.
Not sure what the current favorite Lisp implementations might be for different platforms. Google it.
OO Programming: Get a copy of Java and one of the many excellent tutorials out there. Go to Amazon for a copy of "Design Patterns: Elements of Reusable Object-Oriented Software". Read it cover to cover. Five times. While you are at it, get a copy of "Code Complete".
I know this is expensive, but, in general terms, surround yourself with multiple books covering the various subjects. I never buy just one book. I might buy five to ten books covering the subject from different angles. Sometimes you don't necessarily read them cover to cover, but you use them for reference as you move forward. The 'net doesn't always get you there. Searching for solutions on SO does not necessarily teach you what you need to know in order to understand how you got there. Someone can show you the code you need to write an Objective-C class that complies with a Protocol, but, do you really understand the five why's and how's?
For example, if I need to code an FIR (Finite Impulse Response) filter using a new language or on a new platform I can usually do reasonably well hunting for code examples on the 'net. The important part here is that I already know what an FIR filter is and how it is supposed to work. I have implemented them from scratch in anything from hardware to DSP's. So, scouring the 'net for code snippets really becomes a way to help me discover how to express these ideas in this new language rather than me trying to both learn the language and about FIR filters at the same time. Hopefully that makes sense.
I'd much rather hire a programmer that has depth of knowledge in algorithms, patterns and the general subject matter we might be working on (say, as an example, inertial control systems) and tells me that he or she is not fully versed in the language that we have to work with than one who knows the language in his/her sleep but lacks the depth. You can google the language. You can't google the other stuff.
Sorry, this isn't a complete answer. I'd say, above all, as you learn you really have to feed your curiosity with projects you are interested in. If writing a solution for banking doesn't get you excited it is likely that you are not going to be inspired to learn anything new while forced to write that code. If, however, figuring out how to make a robot walk inspires your curiosity, you'll find yourself coding for sixteen hours at a time trying hard to solve problems and learn as you go.