Well a Christian is someone who lives a good life and tries to help others. No, that can't be right either. A Muslim could do that. Or a Buddhist. Or an atheist.
Then a Christian is someone who believes in God and believes the doctrine of the church. Well, not necessarily. Some people have been brought up in the doctrine of the church and they've not been Christians. They've been scoundrels.
Then what is a Christian? Here is a definition I like. A Christian is someone who knows Jesus, who loves Jesus and who serves Jesus.
Christianity isn't just a religion, it's a relationship. Before the Fall, Adam had fellowship with God in the garden. Then Adam sinned, and sin broke the connection. Jesus lived a perfect life and died to pay the price of sin, to make a way through Christ for man to come back into a personal relationship with God.
A Christian is someone who has realised that Jesus is alive. (Remember, the early Christians had trouble convincing people that Jesus was alive.) A Christian has had a meeting with Jesus in which he has given his life to Christ and Christ has changed his life, and continues to change it.
A Christian is someone who has realised he was a sinner without a Saviour, a sheep without a shepherd. A sheep who has chosen to come through the Door back into the sheepfold. A sinner who has come home.
The good news is that whosoever will may come.