Your major clue regarding the problem is that the headlights, etc. do not work. This obviously rules out anything in the starting circuit such as solenoid, ignition fuseable link, starter key switch, etc.

Let's tackle the obvious first.
(1) a battery can put out 12 volts and still not have enough oomph to work the lights or starter. Measure the voltage with and without the lights on. Does it always show 12 volts or higher? Honk the horn...does it honk? If no, does the voltage drop when you push the horn button? Even a good battery will dip in voltage when the horn honks. If no, go to item 2
(2) check your battery cables. I have had cables corroded at the battery post interface and corroded inside the cable jacket. Take your battery cables off and clamp a good set of jumper cables to the terminals, the plus will go to the alternator stud (or the starter stud) and the minus should go to the engine ground. This takes the battery cables out of the system. Now, turn on the headlights. If the headlights work now, one or both of your cables are crap. If not, something else is wrong.
(3) Is your ground strap to the engine connected? Use one of your battery cables and make a ground between the engine (like the alternator bracket where the negative battery cable attaches) and to the frame. Again, use the headlights as a test.

I will bet on the battery cables as being the problem....but try the others too.

mike in tucson