having problems with my car-

was running properly- i parked car in garage after a long drive and it promptly dumped approximately half a gallon out of the overflow's "vent" or "release".

i attempted to let it cool- refill coolant and move on etc.

however, now the coolant is building pressure, overfilling the tank, and generally not working plus the car would overheat if i actually ran it for long. I must have air in the coolant etc., but why can't i figure out how to fix it?

what is the process you use?