When the moral argument comes up, atheists often say things like "I don't need God to tell me what's moral and what's not, I can be moral on my own." One interesting way to reply to this is to ask "then why don't you already follow the morals He prescribes?"
On the face of it, this seems circular. And the truth is, it is! That's the point! See, moral laws are abstract concepts. A physical reality can be observed by all. I can pick up a rock, and you will agree that it is a rock. I can put it on a scale and show you that it weighs 5kg, and you will agree that it weighs 5kg. I can measure it, I can clean it, I can chip parts off it, we will always agree on all of its physical attributes.
Morality, by contrast, is an abstract concept. I can't place moral laws in front of your eyes, I can't put them on a scale, I can't measure them, there is literally nothing that will force us to agree on moral laws. Thus, our moral views will depend on our goals. But that makes your idea of being a good person, first of all, futile. You're arguing an opinion. Worse than that, it's one opinion against, currently, more than 7 billion opinions. So, on the one hand, great, I'm sure you're very good at holding your own opinions. On the other hand, so is the rest of the human race, some of whom will call you a good person, others of whom will consider you no more worthy of life than a pig. But it gets worse than that, because as much as we'd all like to believe we're good people, the fact is we're not the ones judging. God is. And you do need God to tell you how He will judge you when you face Him.
Let's be honest here: We don't often agree with God's morality. In fact, when we read His word, we are more often than not very offended by Him. Thus, we not only need Him to tell us what's right and wrong, but also to make us understand it. We really can't be good without God.
But thankfully, we don't have to be. God didn't abandon us at the first sin. Rather, He made a way for us to be forgiven for it, and return to His path. He sent Jesus to live as a man, a truly good life, and die, facing the full penalty for sin. After that, He rose to life again. All it takes is to repent, confess Jesus as Lord, and believe He rose from the dead. If you do that, you won't be good, but God will judge you as if you had always been as good as Jesus.