2000 Other Cadillac Models Battery or Alternator

Other Category problem
2000 Other Cadillac Models V8 Wheel Drive Type unknown Automatic 109000 miles

I have a 2000 Cadillac SLS. I recently made a trip to the store and when I came back to go home, my car would not start. There wasn't anything that alarmed me of any kind of problems the whole time that I've had it. When I went to start it, the lights on my dash board started flickering, and you'll here this " click. Click. Click" sound. Then the menu began reading service airbags and service theft system. After being baffled for a second, It tried it again and the same thing happened, except this time I was only getting a 9.5 volts from my battery. I did it again and the damn thing died on my (battery). I had a friend come and give me a jump, and after a while it worked. But after I turned it off to go in the house, it stopped working AGAIN. So my question to you is could it be the battery, or the alternator, OR something completely different? I appreciate your help on this one.