A good place to find rom hacks and rom hack tutorials is pokecommunity.

Also, you can find disassemblies of the old game boy and game boy color games on Github which completely recreate the code of the old games from scratch, allowing for really easy modification. Just download a disassembly and stuff it needs to compile, modify something (say renaming a pokemon or dropping in a different image or moveset) and compile - pretty easy! Then use an IPS patcher to make a patch of your new rom to share with others. :3