Use an OFFICIAL CABLE! I cannot stress this enough. Use a Samsung or HTC USB cable or find someone who has one (some cheap cables have never worked for me when servicing phones through Odin). Put your phone into Download mode, connect the USB cable, wait a few seconds, now open Odin. You should see a COM port. Try it now.
Sent from my SPH-L900. Please hit the thanks button if I helped you out!
If I or others have helped you out, don't forget to hit the Thanks button for us!