If your car has coolant in it but it's still overheating it could be caused by a number of different things such as a bad radiator, clogged radiator, bad thermostat, bad water pump.
The most common cause though of a car overheating when it has coolant is due to a faulty thermostat that is mounted on your engine with a radiator hose connected to it.
When the thermostat is stuck it restricts the coolants flow through the engine which causes the engine to overheat.