PLEASE, use code tags, it makes reading your source easier.
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std::cout << "Value the temperature with Celcius: ";
std::cin >> C;
F = 1.8 * C + 32;
std::cout << "The temperature in Fahrenheit is " << F << std::endl;
One final thing for the OP as well - always initialise your variables - even if you assign something to it on the next line. Not initialising things is a very big source of errors, not only for beginners - but also for some not so beginners.