My jeep wont start cause of my alarm, that i dont have a keypad thing to?

So I went to the lake earlier and when I went to leave, my girlfriend opened the door and an alarm went off. It eventually went off. I didn't even know my car had an alarm. I've only had it for about two months, and I bought it from an individual, not a dealer. Now, when I try to start it, it will only stay on for about 2-3 seconds. What is wrong!?