First make sure your shifter is working properly. Make sure it is completely in park and try to start it. Also push the shifter foward and see if it will start. If not, your transmission range sensor is shorted and will need to be replaced. It is located on the drivers side of the transmission. WHen in park or neutral, it completes the circuit allowing your Mustang to start.
First place the vehicle up on jack stands so you can access the under side of the transmission. On the drivers side of the trasnmission is the Range Sensor.
First Disconnect the connector. Place the transmission in park.