A "prologue" is a part of the story that takes place well before the main body of it--months or years before, usually. It is often used to introduce a few peripheral characters characters and the general world, usually with just enough detail to catch the reader's interest. It is never directly connected to the next chapter; if the events in it are ever referenced, it's almost always very far into the story.
This is separate and distinct from the first chapter. The first chapter, which comes after the prologue if the book has one, usually introduces at least one of the main characters and begins to set up the situation central to the book. It is tightly coupled to the remaining chapters, with direct bearing on most or all of them.
To understand the difference, consider the first Harry Potter book. What Rowling calls "Chapter One" (since her audience is eleven-year-olds) is a perfect candidate for prologue-hood; it consists of two characters mysteriously talking about things that won't be mentioned again for a few chapters. Her "Chapter Two", which starts the main body of the story and which is directly connected to the next chapter, would not make a good prologue, even if Chapter One wasn't there.
Since prologues are different from first chapters, you should not simply call the first chapter of your fic the Prologue; you should call it Chapter 1. Doing otherwise demonstrates that you are, in fact, an idiot and that there is a reason you're not being paid to write original fiction.