How to create junit tests in java

How do you create tests to fully explore the functionality of a program? I see it as the desired result for a certain aspect of a program using false entities to satisfy your needs, but I am unsure exactly how to go about it. I have been given something similar to

/**
 * An Australian Rules football score consists of a number of points,
 * accumulated by kicking goals and behinds.
 */
public class FootyScore {

    private int goals;
    private int behinds;

    /**
     * Constructs a team, initializing their score as 0.
     */
    public FootyScore() {
        goals = 0;
        behinds = 0;
    }

    /**
     * Returns the total number of points, based on the number of goals and behinds
     * kicked, and the number of points earned for each.
     *
     * @return the total number of points earned
     */
    public int getPoints() {
        final int pointsPerGoal = 6;
        final int pointsPerBehind = 1;
        return (goals * pointsPerGoal) + (behinds * pointsPerBehind);
    }

    /**
     * Increments the number of goals kicked.
     */
    public void kickGoal() {
        goals += 1;
    }

    /**
     * Increments the number of points kicked.
     */
    public void kickBehind() {
        behinds += 1;
    }

    /**
     * Returns a string representing the way an AFL score would be expressed by a
     * typical footy commentator, as the number of goals kicked, behinds kicked, and
     * points earned.
     *
     * @return a "speakable" football score
     */
    public String toString() {
        return Integer.toString(goals) + ", " + Integer.toString(behinds) + ", " + getPoints();
    }

    /**
     * Returns whether or not the given team's score exceeds this team's score.
     *
     * @param otherTeam the opposing footy team's score
     * @return true iff this team is losing
     */
    public boolean losing(int otherTeam) {
        return (getPoints() < otherTeam);
    }
}

Which simulates goals in football, but am unsure how to fully test it is working correctly? How many tests would I need? What should each test specifically satisfy? How would you satisfy each requirement with the test. What would the completed test look like, do you call @test then make a new function or a pre-existing one?

What is the proper formality to creating tests?

Sorry if it's confusing, I'll try to narrow down the definition if it is. Thanks in advance to everyone who helps me understand creating junit tests.