An alarm on a new car is only to cover the contents of the car. Your car will already come with an immobiliser and will be hard enough to rob it's only to alert you ( but you might not hear it) and a passer by might look at the car but might not and still what ever was in it is gone.
If it's an expensive and rare car that might get robbed, I'd get a Cobra alarm, with tracker and txt alert but you could pay over 2 grand for a proper one with all the trimmings.
What type of car is it? Does it have remote central locking? If it does another reason not to get an alarm.