There are many denomination in Christianity and they have different views about how to become a Christian.
Generally, there are two main views
1) Some Christians( actually, probably a majority) believe that all one needs to do is believe in Jesus, and His role in the redemption of sins.
2) Some other denominations (for example Catholics) see the beginning of a life as a Christian at the baptism - which can be done after birth or at adulthood.
