Credible is believable. It could refer either to a story or a person. The adverb form is credibly. The noun form is credibility. The opposite is incredible, not able to be believed.
Credulous is gullible. A credulous person is apt to believe nearly anything even with little evidence. The noun form is credulity. The opposite is incredulous, refusing to believe, skeptical.
Creditable is having or deserving credit, praiseworthy. The noun forms are credit and creditability. The adverb form is creditably.
See also Incredible or Incredulous?