factory immobiliser will allow car to fire then cut out, more likely to be starter motor fault or wiring issue. could be aftermarket alarm issue if fitted?
look at the motor itself, check red/black wire plug is connected and nothings loose. if it checks out, use chunky bit of wire to jump from battery +ve to the red/black wire terminal on starter. if it cranks over issue is wiring, switch or alarm. if not, starter is dead. put it in 3rd gear and rock the car, then retry, if that doesnt help give it a light tap with a hammer and try again.
