I think there's a variety of ways. He gave us the Bible, yes - he also gave us consciences, that can tell us right from wrong. He gave us logic, we can use for reasoning things out. And yes, he did give us the church, other believers, who can help keep us on track.
As a semi-Protestant (I'm not a member of
any Christian denomination, but the churches I attend are Protestant, and my beliefs are mostly Protestant) Of those, I'd say the Bible is the most important. Logic and reason is just as important, in a way, but I think it should be used to explain, expound, and build off of Scripture, rather than try to find alternate moralities. (I'm speaking as a Christian here - all you pagans and Satan worshipers can do what you want

) Consciences can be valuable as well, but what you
feel isn't always accurate, and can be tampered with by upbringing or stress or just plain human fallibility; guilt or the lack of it doesn't always mean much for determining morality. And the church, of course, can be wrong. And aside from all of these, I would include personal revelation - spiritual inclinations, ideas, experiences, that you believe are from God, and tell you how to live your life.
It may look like I just shot down most of my ideas, but really, I didn't.

I think you need to balance all of them with each other. Ignoring any one is dangerous, and can lead to disaster - all of them are valuable, and all of them are necessary. Because while it's impossible to lead a perfect life, or make perfect choices all the time, or have perfect theology, we should strive to have best theology and the best lives that we can. We know we are forgiven for our imperfections - but we should seek to be as close to perfection, to Christ, as possible, because that is a large part of our purpose.
Make sense?