Below are some useful resources for various popular systems. If you're unsure what you want to work on we recommend starting with a CHIP-8 tutorial, of which there are many (just Google it).
After that, move to whatever system you want to. You don't need to "work your way up" to it or whatever as many seem to think. (relevant thread). Just make sure you have the basics down first, remember to study the source code of existing emulators (super important) and if you get stuck, ask questions on this server or in the emudev subreddit: https://reddit.com/r/EmuDev.
For potentially more resources on your system, also see pinned messages in relevant system channels.
Note: A great introduction to any system is to read its Architecture of Consoles entry, if available.