Look at your favorite book. Read the first two pages of any book that you've read more than once. Write down what draws you to the book. Is the voice captivating? Does the author use great imagery? Figuring out why you enjoy your favorite book will help you when you craft your opening.
Select a genre. It's important that you decide whether your story is geared toward adult, young adult, or juvenile readers as your audience will affect your voice. Knowing your subgenre is also a good idea. Is your story an urban fantasy---a fantasy set in the regular world---or is it a thriller? Your beginning should reflect the genre and subgenre.
Write or type 300 to 500 words of your manuscript. Find a place where you won't be interrupted and get your words on paper as quick as possible. Add every thought and bit of dialogue that comes to mind. Read your opening and correct errors like spelling and incomplete thoughts.
Polish your opening. If the start of your manuscript includes cliches like the main character waking up from a nightmare or a catastrophic explosion, consider revising. Also remove overused phrases, according to the University of Richmond.
Find a reader. Several free online forums are available to aspiring writers such as Absolute Write (see resources). Look for a beta reader for your opening or post your work directly into a forum for a critique. Always think about each revision suggestion before changing your work. If you agree with the critiques, revise your opening.