I get the feeling you are wearing regular shorts, not cycling shorts with a chamois? If so the whole purpose of cycling shorts is to eliminate seams and material that will cause you to chafe. Get a decent pair, Sugoi, Pearl Isumi or Terry cycling shorts and wear them with nothing underneath. You can also buy a chamois cream (Chamois Buuter, Assos Creme, Butt Butter) or use Desitin or Vaseline to help with chafing. Also, a good saddle (see threads on saddles). Makes a big difference!