Once upon a time.... or In a galaxy far, far away....
Seriously though, just write what you want, there really isn't one way to write one. Re-read some of your favorite authors and note how they started their stories. Then you'll have a better idea of where to begin.
start w/ introducing characters, setting, time, etc.. ex- start with talking about the weather- I was running through the dark, pouring, May rain. Squinting through the thick sheet of rain in frount of me, i could just make out a form in the distance. It abruptly lunged out at me. i attempted a scream, but that was before everything went black. Lol, sorry it is kinda bad, but i spent, like, five seconds on it, it's an ok example of an EXTREAMLY short story :)
Just start writing, think of it this way you can always change it later, this can be the hardest part. Figment.com has some awesome authors that can give you good ideas, like starting with action, or with talking to the reader, or talking to another character sometimes its good if the main character just met someone then your telling the whole background story in the first chapter. This is me: http://figment.com/users/217736-Samantha-Bullock being different is the first part of my first "book" I am going to change the beginning it is kind of a bad beginning but I like the book so far, I will help you if you need any help best way to ask me is on figment. Remember your never to young to write a book!
Your profile doesn't tell us anything about you so it's hard to know how to answer this question.
You need to introduce your protagonist as quickly as possible in a scenario that captures the reader's attention. That will vary according to the genre. If the book needs a prologue (as my own novel did) you introduce the protagonist at the start of Chapter 1.
However, there are BAD ways to start a book and every year the Bulwer-Lytton Award is celebrated with contests looking for bad opening paragraphs. It's not easy to write as badly as that and some of the results are very funny. Have a look at the 2011 winner and you'll see what I mean:
"Cheryl's mind turned like the vanes of a wind-powered turbine, chopping her sparrow-like thoughts into bloody pieces that fell onto a growing pile of forgotten memories."