Pretty much what RAP said. Here's a list of the base essential skills you need to program an emu, in my opinion[in no order]:
1. Video programming[DirectX, OpenGL]
2. Audio programming[learn about different waveforms, how to output sound from files, etc]
3. Binary/bit operations/bit shifting
4. Input programming[keyboard, joystick]
5. Very good understanding of your language of choice
And even then it'll still be hard. I'd recommend starting with Chip8. It's pretty much the easiest emulator you can write. Just know that going from nothing to an emulator is really rough. Once you've written one or two, it becomes a lot easier[and even then, it's still hard].