Actually, it depends on the kind of mod you want. Do you want to add parts? New types of fuel? Change planets? If you're leaning toward parts and modifications of existing ones, then Blender is the starting point. If you want to just modify the existing game, programming. I've programmed in Python for over 4 years, RobotC for a few months (during FTC season) and WindRiver (A C-like language for FRC) for a few months as well. Programming, once you learn one language, you can learn any other language fairly easily. Here's a link for Python: LINKY