There are tons of sites, tons of info out there, you just have to look for it. http://www.google.com/search?q=sega+...en-US:official
Try that for a start. I hope you also have a lot of time available to read the manual on the Motorola 68K family too, some z-80 assembler background will help with sound programming. If you want to try something simpler, start with the master system, it uses the z-80, easy as hell to program for.