Oh, here we go 
don’t really want to burst your bubble, but I would start from finding a good team of at least 20 people, that can help you with a game that big in scale. And be prepared to spend years and years in development.
The thing with games like that is that you can’t be everyone. Meaning, you can’t be a 3d modeler a programmer, level designer, sound designer etc. I think I missed at least a few more roles here, but my point still stands. I would strongly recommend to get good at something that you really like. You like working with 3d applications be a 3d modeler.
There are people that have a wide knowledge in most of this fields, but they work on a much smaller projects. And if it is a programmer I can guaranty that he outsource all his 3d, lvl design stuff.
Again don’t take it personal and good luck!