The exhaust fumes of gas cars contain quite a big amount of water. When you switch off the engine, some water stays behind in the exhaust system. When you start your car, two things happen: 1. The cold exhaust system lets the water condense out of the exhaust fumes (it is the same effect when you breathe in cold weather) 2. The water staying in the exhaust system is converted into steam and blown out of the muffler. You will also notice, that many cars show some liquid dripping out of the exhaust after cold start and during operation. This is mainly condensed water.
Thanks appreciate it was abit worried was thinking it might me the motor , never ownd a petrol car only had diesal before