A little water coming from the muffler for the first few minutes after you start the car is normal. Water vapor condenses inside the exhaust system when you stop the motor, and is expelled the next time you start. If there is a lot of water coming out, and it keeps coming even after the car is thoroughly warmed up, then it could be coolant leaking into the cylinders from a bad head gasket or a cracked block. You can have a mechanic do a pressure test on your cooling system to test for this. He also can test for the presence of exhaust gasses in the cooling system.