The main function in toc corresponds to the main function in C. This function is called when your program is run. So, this is a valid toc program which does nothing:
main ::= fn() {
};
It declares a constant, main, which is a function with an empty body. Note that the syntax for declaring functions is the same as the syntax for declaring constants (it isn't something like fn main() { ... }).
Assuming you have compiled the compiler (see README.md for instructions about that), you can compile it with
toc <your filename>
You will get a file called out.c, which you can then put through your C compiler to get an executable file which does nothing. Congratulations! You've written your first toc program.