Once upon a time ... (Any old Fairy Tale)
It was the best of times, it was the worst of times ... (Tale of Two Cities)
There once was a boy named Eustace Clarence Scrubb, and he almost deserved it ... (The Voyage of the Dawn Treader)
Every good story has to begin somewhere. It needs an opening line that peeks your interest and makes you want to read more. It needs an opening chapter that introduces you to the characters, but leaves you with questions as to why are they like that!!!???
It needs a good beginning.
The question is, however, how do you get a good beginning?
First off, start with the chapter. You can always change the opening sentence later to make it sound more grabbing, but if you write a brilliant first sentence, and then it completely does not fit the book, well, it'll just make you cry. Sure, try to make the sentence as grabbing as you can (for you, if you aren't pulled into what you're writing at the first sentence, you'll never finish. Trust me, you won't) but don't obsess over it, yet.
In writing your chapter you want to do three things:
1. Introduce your main character. This is just common sense. If you wait till chapter three to introduce the MC, your readers will be all ... "What ...? I thought Staci was the MC, who's this Jerald you're now so interested in." Of course you need to do more than just say, "This is a book about a guy named Jerald, he's going to be the MC of this book!" No! you need to let your readers fall in love with him! You don't have to tell every little thing about him at this point, but you do need to let your readers see his core personality, and his biggest quirks.
2. Introduce the Conflict. What is the story going to be about? What problems are your characters going to have to face? Let your reader know! You don't have to introduce the entire conflict, but enough so that your readers can start rooting for your characters.
3. Introduce secrets. Don't give away everything upfront. Let your readers know that your MC is a dragon rider, but you don't have to tell them all the riggamaroll they had to go through to get into that position, yet. Let your readers know that Julia has found a glowing egg in her backyard, but you don't have to tell them that there is a Mr. Fiery intent on hoarding all the glowing eggs in the universe, yet. You can tell your readers everything later. Right now, you're only trying to get their attention.
As for the first sentence, what do you need to look for in it?
1. It needs to fit the mood of the book. If you have a first sentence that invokes fear, your reader will be expecting a scary book. They'll then be severely disappointed when you're book is a happy-go-lucky book about a girl who collects flowers.
2. Make it pertain to either the conflict or the MC, preferably both. If it's about a character who disappears never to be mentioned again only a few pages later, or a conflict that is resolved before the page is over, your reader won't like it.
3. Make it either profound, exciting, or intriguing, preferably all three. Do an observation of your character's life philosophy, or maybe have him ask a question, or start it with some action - but make sure it fits your book!
Remember, the first line and first chapter will probably be what induces your reader to read the entire book. If you fail that, your book, no matter how good the rest of it is, will never be given a second glance.