Overheating

I have replaced the radiator. Brand new just bought a few weeks ago radiator. I was in a wreck and had most parts replaced to new, but when I first drove her she overheated on me in a matter of seconds! Pulled back into the driveway and it spewed out from the front like a water hose spitting at you. Mechanic looked and advised to get a thermostat and new O-ring. I wondered if anyone could tell me how to find the thermostat on my car and how to replace it myself if possible in detail?