The Catholic Church canonizes saints based on their manner of life and on the basis of miracles attributed to them. Holiness is necessary. Note that it is not the saint that grants any miracle but his prayers to God. A saint is a Christian brother or sister that is still alive after his or her death and is still our brother or sister. He or she becomes part of the “communion of saints.” When we accept Jesus Christ as our Lord and Savior, we have passed from death to life.
Here is the long version. Start with 32) to read how miracles are handled.