The Usual First Sentence begins with...

Once upon a time...

Now, change it. You are about to start on your best journey: through your senses, your emotions, and your life. In the same way that you can't simply begin your life knowing everything, you can't start a novel with a first sentence.

Sounds weird?

Your first sentence has to draw people in, and make them want to read your work. Can you do that immediately without agonizing over it, and therefore crippling your chances of going forward? If you can, by all means, write your first sentence and go from there. If you can't, admit that you can write your first sentence later, and just write.

What do these two scenarios have in common? You just need to write. Whether your first sentence comes out all polished the first time, or you decide to write it when you're done with your novel, you simply have to write.

Go and make your mark on literature! Write!