This is the signature (more or less) for a builtin function called strlen that does what you're supposed to do.
Code:
size_t strlen(const char * str);
You probably want instead to prototype your own function, stringLen() or whatever it's called. But that function also has a number of errors; missing semicolon, multiplication for some strange reason, no string being passed in.

I would try to write this program in one file at first until you at least have some code that works. Figure out how to pass in a string to your function. Figure out how to call that function with a hard-coded string, and later progress to using a command-line argument. Maybe read some tutorials or pick up a book on C or C++; calculating the length of a string is often covered pretty near the beginning.