How to create an environment on your computer that keeps your file impossible to be stolen?

I want to do some sort of extremely confidential programming on my computer - yes, I know that might sound stupid, but I still think that stupid way - and I want that no data go to third-party stuff, be sent to Internet ( for example, google store your data), or anything similar. I believe that trying to shut down individual of such things is rarely a proper and trustworthy method, instead I want a ground up solution - just like what you write on your notebook could never be stolen, provided you have keep others from directly accessing it because it is just impossible. Any help or advice.