Could be a number of things. First check your coolant level. If it is low, fill it. Let the car run for awhile so it gets up to temp, and try to spot any leaks. There is a good chance it is a very small leak that is not visible to the naked eye, in which case you'll need to take it to a mechanic.
If your coolant level isn't low, a possible cause is a broken thermostat.
I'm not the smartest guy here, but I've gotta stress, if the car is overheating DO NOT RUN IT.
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