Talking to my computer through Serial

ok so I have been trying to get this modified code to work with no success, lets see what the problem is:

String readString;

void setup() {
Serial.begin(9600);
Serial.println("serial test 0021"); // so I can keep track of what is loaded
}

void loop() {

while (Serial.available()) {
delay(2); //delay to allow byte to arrive in input buffer
char c = Serial.read();

}

if (readString.indexOf("hello") >0) {
Serial.println("welcome, Doug");

readString="";
}
}

When I say "hello" it should say "welcome, Doug"