Reflection of data

How do I get any object and its private field read and then returned

public class Person { private string _password;

    public Person(string name, Gender sex, int age, string pwd)
    {
        Name = name;
        Sex = sex;
        Age = age;
        _password = pwd;
    }

    public string Name { get; }

    public Gender Sex { get; }

    public int Age { get; }
}

public static string ReadPrivateField(object obj, string fieldName) {

    }