your myValue is a byte
you have initalized it with A0, which might be nothing else than 14.
you add 8.
you do that over and over again.
after some time (255) it rolls over to 0
so what.
To get the data of an analog input, Arduino has the function analogRead(): analogRead() - Arduino Reference
There is often a very small example or links to examples.
You misunderstand this statement completely. My guess, you think it "connects" the value with the data coming into the port. It's much more limited than that. It actually just creates a new variable and gives it the initial value of the pin number of the port, not the data.
You should make it a const, and give it an appropriate name if you want to do it: const byte analogPin = A0;
then you can use int analogValue = analogRead(analogPin);
instead of int analogValue = analogRead(A0);