The SEGA 16 bit home brew scene is growing all the time but it's still fairly small, the only way to change that is for more people to get involved.
If you're interested in SEGA development then hopefully this page will give you some pointers to the current development tools, projects and forums available. Links to everything mentioned can be found at the base of this page.
So where would you start? First I'd join some of the excellent communities out there, there are some very knowledgeable people happy to share what they know. SEGA-16 is a very active community, and SpritesMind a community of developers.
Next you'll need a tool chain, there are now options for BASIC, C/C++, and ASM programmers. I'd highly recommend learning 68k Assembly Language if you want to make games for the Megadrive/Genesis. It's not actually that difficult, Easy68k is a good place to start when learning the basics and structure of the language. When you are ready to learn the specifics of the megadrive, read as much of the documentation as you can. Understanding how the machine works is essential to creating games for it regardless of the language you use to program.
Use the tutorials! The one by Big Evil Corp will give you almost all the code you need to write a simple game. You will also need tools to make resources such as the 16-bit graphics, these are thin on the ground but I recommend Graphics Gale for creating pixel art and SGTD is also good for creating tiles.