I need to know what to do when I have a vehicle that starts and goes to a certain rpm and then quits.

My car just started doing this two weeks prior. I would go somewhere and come back to the car after going into a store and would come out and start the car and it would die, but then would start up again. It became more frequent. Then one day I went to start the car and it would start up, hit a certain rmp and then die. Every time. I have never encountered something like this, so I am clueless.