You already declared int sum outside the loop, so you do not need to re-declare it anymore. You should also try to declare all your variables outside any sort of loops.
You are adding sums to total, but user's input goes into int num, not int sum. But you have the right idea with the total += sum, you're just adding the wrong variable. And you should add it AFTER the user has inputted the value.
float total = 0;
for(int count = 0; count < 7; count++) // Loop 7 times
int num = 0; // moved declaration of count and num to keep their scope smaller
std::cout << "enter a number" << std::endl;
std::cin >> num;
total += num;
std::cout << "the sum is " << total << std::endl; // " instead of ' and total instead of num