scroll down this page to circuit 5 http://home.cogeco.ca/~rpaisley4/LM555.html
this should be what you are after? you can bridge the trigger (switch) and leave it because I think you want to work it from your power supply and it will still give just one pulse once the power is on,until it times out.
If I recall correctly the input voltage is 9v but your 12v will be ok.
you can adjust the timings/pulses etc just by the values of the resistor and capacitor , there is a calculator on the page for you to do that if you need to.