How do I fix a horn honking problem caused by opening a drivers door.

What is your question?

when I opened the door on the drivers door the horn starts beeping on a regular basis like its an alarm system.Does thei car have an alarm system ? and if so how do I fix it or dis-able it. I got this car 2nd hand with no instructions period. If it is not an alarm what is it and how do I repair it. The horn only stops when the battery runs down or is unhooked.