If it is the stock alarm system, check the sensor for the hood - it looks like a plunger on the passenger side near the shock tower. Those go bad and will set off the alarm because it will read the hood being open. About $35 for the sensor. Quick and easy to exchange.
One way to tell if something is wrong with the stock system is when you lock the car. If it chirps twice then it is telling you a circuit is open. If it chirps once then all the sensors are reading closed.