The word Christian has been thrown around in the media a lot lately. Simply put, the word Christian means "little Christ" or Christ-follower. It was a derogatory term given to the followers of Christ in the first century. It was intended to mock them - these silly people following this man who ended up dead. The first, most basic message of the Christian movement was "repent and believe the good news of Christ."
It was preached by John the Baptizer, Jesus, and then the early apostles. Repent and believe. So to become a Christian, a person must first understand they are a sinner. They don't need a crutch - they need a stretcher - and the stretcher is Christ. So they repent of themselves and their sin, turn to Christ and believe He is who the Bible claims He is. They ask Him personally to come in to their lives and dwell in them through His Spirit.
The second message of Christ was, "Follow me." So, for those who repent and believe, the rest of their lives they are on a journey learning to walk and follow Christ. As time goes on, people around them should be able to see Christ slowly being formed in them - and they over time slowly become "little Christs." Never perfect, but always being formed by the One in whom they believed. Stumbling, but getting back up. Learning to trust God more than trusting self. There is no Christianity without repentance and belief. And to be a Christian is to be on a journey of becoming like Christ.