Starter 1991 Isuzu Stylus Manual Trans

I was told and it was confirmed that I need to replace my starter on my 1991 Isuzu Stylus XS Manual Trans. But I would first like to know why all of a sudden, the car would try and start on it's own? The moment you attach the battery, the car tries to start on it's own. Do I have a short somewhere else or is my problem the starter going bad. I have not looked yet but, where exactly is the starter located? I'm going to attempt to do this myself. But before I do, does this car come equipped separately with a solenoid, and if so where is that exactly located?