To get started making ROM hacking tools, you'll essentially be making a tool to automate a process that could be done very slowly with a hex editor.
To that end, what you'll really need to learn first is how to read/write bytes to a file. Once you understand that, and what changes you actually want to make to the ROM, the programming should be obvious. Of course, you will need to read up on the hardware for the ROMs you'll be working with. Important topics would include how pointers are stored, how colors are stored, etc.
