How do i diagnose for 1995 Camry that intermitantly will not start?

When it is No crank, no start, I do have head lights and dash light but no indication lights other than seat belt and door. no clock no radio. When I turn the key There is a click somewhere around exhaust manifold and top of engine where the spark plugs are. this problem happens very sporadically. I can drive for a week and be fine then nothing. or drive once then nothing. sometimes will start after a dozen or so tries other times not. When car is running I do have properly functioning back up lights.