How does the church become a basilica?

A basilica is a designation that is given to a church by the Roman Curia (the governing body of the Church). The designation basilica is typically given to churches of particular historical significance to either the Church at large,or more likely the local area where the church is located. It is important to note that a basilica is not necessarily a cathedral, a cathedral being the seat of a bishop. For example Saint Peter's Basilica in Rome is not a cathedral, it is just an important church.