Painting a Steel Door on Our Pool House

Last year my step dad found a very nice steel door that someone had thrown into his roofing company’s dumpster. We needed a door for our pool house so that we could keep out any kids who curiously wandered nearby. The door however needs some work. It is currently an ugly blue with some spray paint spots of magenta (Which is probably why the door was discarded since it is physically in perfect condition). I am just now sure how to go about painting a steel door. Do you have to do anything to prep the surface, and do you have to use a special kind of paint? I want to do it right the first time so I need to find out the correct procedures.