Car tries to start but then dies....

First and most obvious, make sure you have gas. Unfortunately, that's the end of the easy answers. If it starts for a second then dies, it's usually electrical in nature. During start-up cars usually have a set of parameters they go by. Once the motor catches, they start to read various sensors and go from there. Check all the connectors and make sure they are making contact. Have you changed anything or done anything recently that might affect this? In our shop we would attach a scan tool and read the incoming data to find the anomaly. It's a little harder for the DIY'er.