Engine Cooling problem
2003 Dodge Stratus 4 cyl Two Wheel Drive Automatic 90,000 miles

My car is leaking coolant and i just put in a new water pump. What are the symptons of a bad thermostat, will a bad thermostat make coolant to leak cause when i drive it the tempature gauge goes a little abover a quarter and when i shut it off its bubbling where the coolant is supposed to be, coolant resivoir i think its called and i can hear steam coming out by the radiator if coolant an't going into the engine and its all going into the radiator is there a overflow thing that makes it leak out, please i need some help can't figure it out