This is a great introduction to emulation in general.

While I was trying to figure out how to write my first emulator I thought it would be simpler to start with a Game Boy Advance or an 8-bit processor like the 6502 or the PICO-8.

After a few false starts it turned out to be simplest to write a simple x86 emulator since I already had all the tools locally (assemblers, disassemblers, compilers, etc.) and a working reference environment (my laptop).

Here's the walkthrough I wrote for building an x86 emulator in JavaScript that you can use to run simple C programs compiled with GCC/Clang:

http://notes.eatonphil.com/emulator-basics-a-stack-and-regis...

Now that I did this though I'm still trying to get into a GBA emulator.