To come up with a new programming language, you first need to come up with its formal grammar. Using the formal grammar you decide things like e.g. whether your true/false variable will be called bool or boolean or Boolean (over-simplified example).
Once the grammar is in place, you write a program in a known programming language which uses the rules of the grammar to take lines of code as input and produce machine executable code as output. Such a program is called a compiler. The machine executable code is usually specific to the machine it will run on e.g. if its an intel processor, then your compiler needs to produce intel compliant assembly language.
So the most essential Computer Science courses that you need to take before designing your own language are Computer Architecture, Automata Theory and Compiler Construction, then you need to learn a few things about the assembly language of the hardware on which you plan to run your programs and finally a low-level programming language like C which can help you write your new compiler.