8BitWorkshop is perhaps the most approachable way I have seen yet for beginning retro game development, specifically for 8Bit systems such as the Atari VCS/2600, various arcade systems and now the Nintendo Entertainment System.
8Bit Workshop is a complete IDE and emulator that runs entirely in the browser. You can launch it directly by clicking here. 8Bit Workshop supports the following platforms:
- Atari 2600
- VIC Dual
- Midway 8080
- Galaxian/Scramble Arcade
- Atari Vector
- Apple ][
In most systems you can code directly using C or assembly language. It also comes absolutely loaded with examples in a variety of languages. Additionally they have several supporting books Making Games for the Atari 2600 and Making Games for the NES.
Even better, the entire thing is open source under the GPL v3 license on GitHub. You can also download several samples to get started right here. Finally, version 3.4.0 was just released adding NES support, a new book and more.