How to insert String parameters into a PreparedStatement?

How can I create string and then put the String into preparedStatement, or is this same as using Statement ? When I execute the below code, I've got the following message: "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '?' at line 1".

import java.sql.*;
import java.util.Scanner; 

class firstApp2 {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        System.out.print("Enter first name: ");

        String firstName = scan.nextLine();
        System.out.println(firstName);

        firstApp2 test = new firstApp2();

        test.selectByFirstName(firstName);

    }

    public void selectByFirstName(String fn) {
        //System.out.println("test");

        try {
            String dbUrl="jdbc:mysql://localhost:3306/student";
            String username="root";
            String password="root0216";

            //get a connection (step1)
            Connection myConnection=DriverManager.getConnection(dbUrl, username, password);

            String sql = "Select * from students where firstName = ?";

            //create a statement object(step2)
            PreparedStatement preparedStatement=myConnection.prepareStatement(sql);
            preparedStatement.setString(1, fn);

            //execute Query (step 3)
            ResultSet myResultSet=preparedStatement.executeQuery(sql);


            //process the resultset
            while(myResultSet.next()) {
                System.out.println("Student full name :" +  myResultSet.getString("firstname") +" "+myResultSet.getString("lastname"));
            }

        }catch(Exception e) {
            System.out.println(e.getMessage());
        }

    }



}