5150 II head with Line 6 Spider IV cab?

I have a very limited knowledge of ohm's law and I have no idea how to hook up this rig. I don't know how stereo or mono cabs or heads work so I've just been plugging both cords in and keeping it at 16 ohms to make sure I don't blow my speakers. After a few hours of research and asking friends I still didn't find anything useful. How do I hook this up without damaging anything?