If you already know C then all you need is SGDK
SGDK - A free and open development kit for the Sega Mega Drive - GitHub - Stephane-D/SGDK: SGDK - A free and open development kit for the Sega Mega Drive
github.com
Ideally you'd use assembly for megadeive programming but unless you're planning on moving 100s of sprites on screen at once, SGDK will do fine.
Good luck with your quest