serial monitor problem

im new using arduinos , whenever use serial monitor displays value around 300 , drops 170 , stays way  every time try use potentiometer or photo resistor.

i have been using sketch in arduino library

void setup()
{
  serial.begin(9600);
}
void loop()
{
  int sensorvalue = analogread(a0);
  serial.println(sensorvalue);
  delay(1);       
}

are sure using right analog pin. seems me a0 floating.

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