Why does a car stall as soon as you turn it on?

Without any information (year, make, model, engine size, symptoms, etc etc...) I'm really guessing here. Possible ignition problem, possible security system issue. One idea is to check on the dash and see if the security light is on and if it is then it's off to the dealer for repair & reprogram. The security system aside, three things come to mind, but this is, like the first , just general stuff. It could be a fuel problem. Clogged filter, pump problem or fuel pressure regulator problem. It could be a "set up" or "tune up" problem, which has a lot to do with the control of the aforementioned fuel system - and the ignition system - which is the second area where the problem may be. Lastly, a big vacuum leak could allow the vehicle to start and then die as it is trying to come up to idle. (The leak allows the negative pressure in the intake to suck too much air past the carb - if that's the kind of setup the vehicle has.