u/Live-Ad3442

Hi guys I need help identifying what might be causing my car to overheat. I just picked up the car like two weeks ago and I am still new to all this car stuff. The car would overheat after being driven for a bit, I noticed that the temperature gauge would fluctuate to hot then halfway to hot. The coolant reservoir was also bubbling when I stopped the car and checked. I read it could be a head gasket, thermostat, or even the radiator cap. I need help or advice to find what exactly is causing it so I don’t buy unnecessary parts. It’s also B20B engine.