How can I do a C++ program that converts simple C programs to assembly programs?
This is a very complicated task. The first step would be a parser that could correctly interpret and understand C++ language, reporting any errors that it encountered. Then the program would need to be able to convert those statements into a series of available machine language instructions. Finally, if you were outputting assembly language source, you’d need to be able to write the appropriate statements or produce actual machine codes if you wanted to output object code.
If you need to ask how to do this, you might consider an easier task for now.