2002 Toyota Camry

Toyota camry- 2002- automatic- 4 cylinder- about 90,000 miles on it. The factory alarm sets off on its own and it doesnt allow me to turn on the car. I've tried resetting it with the key on the driver door, but the key only turns to unlock the doors and doesn't turn back the opposite way (to lock). What causes it to set off the alarm? And how can I fix it? The trick that has worked so far is to turn the key a few times on the door (I lock it while I'm inside the car) then I press the red button (alarm button) on the side and while holding it down I start the engine. That seems to work most of the time. Please Help!
Thank you.