One way to become a Christian is by accepting Jesus Christ as your lord and savior. Though any church you ask is going to say there are a few more hurtles then that. The real question at that point would be do I want to become a Catholic, or a Protestant, or a Mormon, or any of the other thousands of other christian churches. You could just pick up a Bible, still less than a hundred different versions, and practice without a church; either way, the first sentence is the important part.
- Another Answer
A person can approach a Christian Community, a Church, and seek to join. After a period of instruction (sometimes called Catechumenate, from the ancient church, a time of being formed in the Christian faith), they can profess their faith, usually by saying the Creed, and seek to be Baptised.
The Early Church had a special ceremony at Easter for baptising new Christians. Most Churches still do this at the Easter Sunday Church Service.