Global integer variables are initialized to zero. So A, B, C, D, inputA, inputB, inputC, inputD, and input are all 0.
It doesn't look like you have gone through any of the tutorials. Your code has numerous problems.
teoditomozzo:
Serial.begin(9600);
This should only be in void setup(), not in loop().
teoditomozzo:
input=Serial.read();
You should not be calling Serial.read() unless Serial.available() is greater than 0.
teoditomozzo:
if(input == A);{digitalWrite(2, HIGH);}
inputA = 1;
Your if-statement does nothing because of the semicolon. The correct syntax might be:
if (input == A)
{
DigitalWrite(2, HIGH);
}
Lines like "inputA = 1" and inputB = 1" execute every time. Did you mean to put them inside of the if-clause? Also, in your if-statement "input == A" doesn't make sense to me. A, B, C, and D are all initialized to 0. Why are you always comparing the incoming byte to 0?
You need to clean up at least those problems before you can go much farther. Regardless of what you see the LED doing, I am very certain this code isn't doing anything you expect it to be doing. This is a great example of where comments would be helpful.
e.g.
if(input == A);{digitalWrite(2, HIGH);} // if the letter A was typed, turn on the LED on pin 2
Comments like those help to indicate to someone else that the code you wrote doesn't do that.