The best way to NOT start off is, "Hello, you're going to hell if you don't repent." While that sounds silly, I've seen a lot of christians start off with that attitude, if not those particular words.
In other words, remember first that Jesus built this church on love, not getting right. Getting right comes from a love for God, and as a response to God's love to you.
In other, other words, the best way to argue with an atheist is to not argue with him. If you start out by trying to persuade him, you have already lost.