If you want to make your own language, you should start by looking at some of the other programming languages around. Many are not the same -- take a look at Scheme, Ruby, Forth, Joy, Lisp, Io, and maybe some esoteric ones like Ook!
You don't need a compiler; you only need an interpreter, which is easier.
If you create an RPN calculator, you are not far from creating a programming language. This happened to me accidentally. Looking up what "RPN" means is up to you, though.