#include <Servo.h>

Servo srv;

void setup() {
  srv.attach(9); // Attach to the servo on pin 9  

}

To use a servo, you need to call attach() function first. It starts generating a PWM signal controlling a servo on a specified pin. On boards other than Arduino Mega, use of Servo library disables analogWrite() (PWM) functionality on pins 9 and 10, whether or not there is a Servo on those pins.

void loop() {
  Servo.write(90); // Move the servo to 90 degrees
  delay(1000); // Wait for it to move to it's new position
  Servo.write(0); // Move the servo to 0 degrees
  delay(1000); // Wait for it to move to it's new position
}

Note that you are not guaranteed that the servo reached the desired position, nor you can check it from the program.