A good place to start would be investigating how to build a command line parser, something that can tokenize and interpret the inputs. There are tools that can help with this like ANTLR, or you might like to try building your own.
Once you've parsed the inputs you need to decide what actions to take - launching processes, piping between processes, redirecting output - and making those system calls.
If you're just after a more powerful shell rather than interested in building one, give PowerShell a try.
