Re: 2007 Impala Overheating
With all due respect to MN Ranger, I think he gave you gearhead information. Hopefully you will heed the warning on the cap "never remove hot", you can get seriously burned fooling around with cooling systems if you don't know what you are doing.
With the car cool, simply fill the radiator to the base of the filler neck as instructed. Then put the cap on properly, and fill the overflow tank to at least the full cold line, and probably somewhere between the full cold and full hot line. As you drive the car (through some heating up and cooling off cycles) air will be expelled from the cooling system into the overflow tank, and be replaced by the fluid in the overflow tank when the car cools off. The overflow tank may need to be refilled.
Now that will solve the symptom (no coolant), but not the problem, which is you have a leak somewhere. You need to find someone (friend, repair shop) with a pressure tester, and find the leak. It could be internal to the engine as MN Ranger stated, or it could be a hose, radiator, heater core, etc. The leak may not be evident because if it's in a hot place (at the engine) it will evaporate pretty quickly.
Also, not too many cars have "low coolant" sensors, your usual indication is the temp light, at which point it's too late as the engine is overheated and possibly damaged.