1996 Mazda Protege Battery issue or something else?

I drove my car to work with no problems. I left for the day and my car wouldn't start. I didn't hear anything when turning the key and nothing turned on. The radio didn't work nor did the digital odometer or the radio or headlights. After a friend jumped the car it ran fine. I got it home turned it off and started it up again without an issue. The car has started with no problems the past couple of days. I do notice that when A/C is on the car drops below 500 RPM's and starts shaking. Also when I am sitting idle sometimes the TAC drops below 500 and the car shakes. I'm not sure If I need a new battery or alternator. Either way I don't think the car is getting enough power.