The short answer is that a disciple is a lifetime apprentice (learner) of Jesus Christ.
The first step in becoming a disciple is accepting Christ as your Lord and Savior and confessing your sins to Jesus. The next, or continual choice of being a disciple, comes with growing daily in Christ's teachings. This growth occurs daily, sometimes moment by moment, action of dying to self and being obedient to the Holy Spirit's adjustments and assignments.