writing a function

Write a function that takes as a parameter an string and returns the number of times each lowercase vowels apepars in it (a, e, i, o, u). The function prototype should look like:

void countVowels(string str, int& aCt, int& eCt, int& iCt, int& oCt, int& uCt);

Also, write a main program that tests your function by allowing the user to enter in a string and is passed to the function.
PLease help i can not figure this out.

The C++ programming course is just stressing me out I get it but then again i dont