“How do we avoid sin and live godly without the Law?”

If we’re not under the Law, how can we know what sin is? If we’re not under the Law, do we have to put in effort to obey it? If God knew us before we were born (as we read in Jeremiah), then why would He create us, knowing He would send us to Hell? What is my best elevator pitch next time my dad calls me and asks about my faith? Do we Christians have a deceitfully wicked heart or a new heart?