Look at the commented lines of where I edited your code. I did the first statement for you, look at http://www.asciitable.com for info on the rest...
Code:
#include <iostream>
using namespace std;
int main()
{
int A; //int not float
cout << "Enter any character then press enter:" << endl;
cin >> A;
if (A>=65 && A<=90) //because ascii characters 65 through 90 are Upper Case letters.
cout << "The character you entered was a Upper-Case Letter" << endl;
else if (statement2)
cout << "The character you entered was a Lower-Case Letter" << endl;
else if (statement3)
cout << "The character you entered was a Digit" << endl;
else
cout << "The character you entered was none of the above" << endl;
return 0;
}