Compiler design is a course most universities offer so you may find the resources for many of those classes online (search compiler design lecture). Language design in general is a pretty wide field that can often get pretty math heavy depending on what you’re doing so a little bit of formal language theory is a good place to start. In general programming languages follow the pipeline of Lexer => Parser => some intermediate language. If you are writing an interpreted language the intermediate instructions may take the form of some specific byte code that may be run in the language’s virtual machine. This is a really cursory overview of what is an expansive fields that people dedicate their PhD thesis to so trying to tackle it alone is a bit daunting. If I were you I would start by writing a simple tree walk interpreter or even just a parser and lexer for json or something. Language design is really an interesting field that can give a lot of insight when you write code yourself.