1991 Sentra SE-R Starts Intermitantly

My Nissan does not start consistently. At times it fires right up, and others I can turn the key for 20 minutes without any luck.

When I turn the ignition on for the most part nothing happens, no cranking, no starter whine, nothing. Its as though I didn't try. At other times, I hear a loud click yet nothing still happens. Then out of the blue it will start.

Lights, radio and accessories work fine. I must get this fixed. Please help