In order to silence the beep of a car alarm, you must have the vehicle key. Some car alarms are sensitive, going off because of wind, loud noises or people who pass by, according to The Unofficial DMV Guide website. The vehicle's alarm will sound for what seems like an eternity, but most go off within 10 minutes. Most alarm systems can be quieted with use of the vehicle's key fob, but if the battery in it is weak or dead, the original key will have to be used instead.
Hit the "unlock" button on your key fob. The button usually has a picture of a lock in the unlocked position as opposed to one in the locked position, which locks the vehicle and sets the alarm. Move closer if your fob isn't working from a distance -- your battery may be getting weak and need replacing.
Put the key in the door to unlock it manually. This procedure may work if the key fob does not.
Open the door. If the alarm is still beeping, put the key into the ignition to turn the car on. This should stop the alarm if the other methods were ineffective.
Vehicle alarm systems differ by manufacturer. You might find information in your owner's manual to completely disarm the alarm system, although you probably need a trained mechanic to do so.
If the alarm constantly goes off, take your vehicle to a professional as soon as possible. This usually signifies an electrical problem.