Unlock and/or start car
The first thing you should try to do is to unlock the driver’s side of the car. Most cars have a safety feature which turns the alarm off if the original key is put into the driver’s side of the car and the door is unlocked.
Other cars require you to unlock the door with the original key and then sit in the car and turn it on.
Doing both should most probably turn off the alarm and allow you to drive away safely.
Remove alarm fuse
The next best thing to do, is to remove the car alarms fuse. Now if you had the alarm put in after you bought the car, chances are it won’t be marked in your cars guide book as to what fuse is going to turn the alarm off for you.
In this situation, the smart thing would be to remove all the fuses one by one, until you come across the one that powers the alarm.
Removing all the fuses won’t do any harm to your car, as long as you place them back in the same place you took them out from.
The last option you have is to disconnect your car battery. In some cars, doing this would end up resetting the car alarm, so if you have the front door unlocked when you do this, the alarm won’t go off and you will be able to drive off, without the sounds of the alarm blaring in the background.