This might sound strange if you're not already familiar with it, but the most important chapter in your book is the first one. The most important paragraph in your book is the first one. The most important sentence in your book is the first one. Getting the point? More so than your climax, more than the big twist, more so than the neat bow you wrap the book up with. The first chapter is more important than any of that. Why is it more important than the climax?
Because that's all you get to entice your readers. One chapter. And in the cases of many, you get one paragraph. If your reader is bored in that first chapter, they're more likely to put the book down and never look back. Yes, there are those who give longer before throwing the book away, but the majority of readers are a fickle beast. They want results and they want them now.
Honestly, the first chapter is one of my biggest sore points in writing. More than my dyslexic issues and info dumps, I struggle to find the right balance. Lots of people do. Maybe it's because we put too much pressure on ourselves to have that oomph right out of the gate or maybe it's just lack of experience. I'm not sure what it is, but if you struggle to have a good first chapter you're not the only one. With every book I write I feel I get better, but I don't know that I'll ever have that perfect first chapter right out of the gate.
One time, with my Freelander re-write, I had a real editor read it. Not the version I have on here, but one of several versions ago. I've rewritten that chapter at least 10 times on its own. The editor said I had a good hook, but it's buried under 3k words of boring crap. So I re-wrote it again. And again. And again. This is not uncommon and don't live in fear of having to put more work into this single chapter then you do any other.
Your first chapter must introduce the story but it also needs a strong hook to reel in the fish. You want your reader to pick the book up and not want to put it down, not even for a minute. They'll put it down because life insists they do, but they're not going to want to. That's the sign you're doing things right.
There are editors and publishers out there who will give you 1000 words to get them to decide to read the rest of your MS. If you're like me, 1000 words is nothing! My chapters are regularly in the 4-6K range. There are some who will give even less than that. They also aren't going to be happy if you take that 1000 words from anywhere but your first chapter. If your climax is amazing, great. But don't send that to the prospective agent. If your MC dying from cancer is the most moving part of the story, awesome. Don't send that to the agent. You send your first chapter and only first chapter, up to however many words they ask for, plus whatever extra you need to finish a thought. Editors/publishers will allow you to finish that sentence even if it goes over by a couple of words.
That doesn't mean you have to reveal the big bad in that first chapter, but you need some kind of hook that draws the reader in. There are a lot of opinions on how not to open your first chapter. Here's a few of my "don't and do's."
I've already discussed the alarm clock method and the dream sequence and why they're not good ideas to start with. I won't repeat them but they very much fit on this don't list. There's also the weather. Don't open with the weather! Nobody cares what the weather is like. "It was a dark and stormy night" is regarded as the worst opening line ever. It is often mocked and ridiculed for a reason.
If your opening is a character waking up and getting ready for school, why not just open up at school where the action hopefully is? Why must you subject the reader to their breakfast, hair curling, and other mundane activities? Now if those activities show the world ending, then okay, but if they're just your typical act of getting dressed then spare the reader. Find the closest point to your first bit of action and start there. One blog called it the act of starting the book as late as possible. Obviously you're not starting it right before the climactic scene where the hero destroys the ultimate bad, but there's a better place to start then I yawned and sat up in bed after a great night sleep.
Leave the backstory for later. Let the reader get to know your characters before you tell us their life story. Same goes for the world history. These are both things I'm very guilty of. Give the reader enough we care about the world and the characters but don't spell everything out for us.
Those are some don'ts, here's some do's...
Give us the MC! Let him or her come out of the woodwork. Some people say introduce the MC within the first 4 paragraphs. This gives us the who of the story. If you're not introducing the MC then why the heck is your chapter starting there? Nobody cares if the villain is hosting a Villains Anonymous party unless the book is about that villain. Open with who that book is going to be about! "But Prisim, what about Harry Potter? That opens with Dumbledore!" True, but he's dropping off a baby Harry Potter so it's still technically introducing us the MC. It also gives us a great hook and question in the boy who lived. We want to know about this boy who lived. So there are exceptions to the rule, but unless you're positive opening with someone who isn't the MC is a great decision, try to have the MC in that first chapter so we know who this book is about.
Make the reader care. I can't stress this one enough. If the reader doesn't care about your MC why are they going to bother devoting their time reading his or her journey? Get the reader to bond with your MC immediately.
Set goals for your first chapter. Think of the who, what, why, and when. By the end of the first chapter, give us those. At least enough of them we care to continue.
And above all . . . don't be afraid of having to re-write it. In all honesty, you probably will have to re-write it and that's okay. Do the best you can at first and sweat the rest of it later.