Use a loop?
'Cos it's an assignment, I'm not going to give you the code. I'll explain what I'd do though: loop through fifteen times, asking the user for input. If at any point the user decides he doesn't want to continue, simply break out of the loop.
To put data in a specific part of an array:
for (int i = 0; i < ARRAY_SIZE; i++)
{
cin >> my_array[i];
}