1999 Chevy Truck Problem with reading the codes

With business slow and the economy so rough I could really use the help right now. My biggest concern is its something serious, and right now I cant afford going to the dealership so I need to try and fix this myself. Im quite experienced with automechanics, and I have a good understanding of electrical and so on, but I am not familiar with these newer diagnostics and emission systems. As I said previously ALL info and suggestions accepted and appreciated. I have looked through a haynes manual myself and they say its called a diagnostic connector, but isnt anymore specific about fuse location or reason that when a tester or emissions machine is connected that it wouldnt display any codes. I am at a loss Thank you all in advance.