Comparable Class - Create Priority Queue For Airline Passengers

My assignment was to create a priority queue for Airline Passengers. Here is what I have done so far:

//Driver

package priorityqueuestandby; 
import java.util.NoSuchElementException;
import javax.swing.JOptionPane;
public class PriorityQueueStandBy {
public static void main(String[] args) {

[Code] .....

So the part that I cant figure out is:

When a standby passenger is to be enqueued into the priority queue, it must be done so that at the moment of each dequeue operation, the item at the head of the queue is the standby passenger with the longest longevity, and also so that passengers with the same longevity are dequeued in a first-come-first-served fashion.

he says that we need to "Make your program so that it extends Comparable and implements the compareTo() method properly..."

So I was looking at the Comparable class and I could't find a compareTo() method... I am not confident I know how extends works either. I am assuming I need a new class if I am going to be extending another class. Right now I am taking in longevity as a String and converting it to an int because my last ditch effort is going to be to set up a loop that will organize longevity into a/an circular array based on the size of the incoming integer.