Engine Overheated!

The other day my engine started to smoke and it was coming from the coolant reservoir. The coolant inside was also boiling. I read in a manual how to check the thermostat. It said that if the top hose to the radiator is cool that the thermostat is closed shut. somebody also told me that the coolant in the radiator should go down when I push the gas pedal but it goes up. It gets hot even quicker when the A/C is on. I've never flushed the radiator and I think that sediments might have gathered at the bottom. Can somebody give me some insight on what could be the problem?