To be faithful is to be filled with faith or trust. Religions have been made on this question. Faith is usually gained by paying attention to the perfection on beauty or sychronicity in a thing. When this outweighs the negative, faith develops.
If you are speaking of being worthy of faith or trust, that is gained by striving toward perfection in a relationship. Be one for one. Be reliable.
Follow His word, put Him first, love your neighbor as yourself, follow the ten commandments, pray and worship Him, be baptized in Jesus' name and filled with His Spirit (with the evidence of speaking in tongues.)
When a person walks consistently with God, in humble service to Him, he or she can be called "faithful."also, that faithfulness is associated with fearing God. The better we truly know God, the more we will want to imitate Him (Ephesians 5:1).