Battery or loose connection to the battery. Best way to know is when the car is making the click, turn the headlights on and try to start it. If the lights go out when it's clicking, there's something loose, usually it's corrocion on the ground terminal. Poor some Coke on it for a quick cleaning, and tighten it down. If the posts are tight and clean already have Advanced Autoparts or Autozone run a free diagnostic on your charging system. It's free and pretty much tells the tale every time.

good luck.