Desperate for help.(Beginner/Noob question)

String x; is not valid, string x; is, C is case sensitive

Yes, it is. But the Arduino doesn't have a string type or a string class. It does have a String class.

 if (Serial.available()>10)
 {
   x=Serial.read();
 }
}

If there are 10 ore more characters available to read, read one of them. How is that working for you? Never mind. I already know.