I've had a few questions lately along the lines of - 'how important is the first chapter?', 'how do you know where to start the book?' and 'should I give the background information for the story up front?'.
The first chapter is where the author has the opportunity to grip the reader's attention. Get them involved so they know that this is a story that won't be putting down real soon. The first chapter should introduce your main character/s in a situation of conflict. And that situation should announce the theme of the book.
The first chapter should start just before a major scene or the build up to one. For example, in my first book the main character finds himself on the wrong side of his favoured skill, interrogation. I introduced the main character in a situation of conflict and in a seemingly impossible circumstances that he needs to escape from.
A lot of first-time writers want to start their book with background information that they think the reader would need to know. I tend to think this is because authors spend so much time developing their character/s that we want to impart all of that information to our reader straight away. Not necessary. A paragraph here or there throughout the book in the narrative or through dialogue can fill in the gaps on background information that value-adds to the story.
The first paragraph needs to captivate or hook the reader. Give them something that spikes their interest to keep on reading. Last thing we want to do is bore the reader - that applies to the whole story, but to do it in the first paragraph can lose a lot more potential readers. I also think it is important to introduce the main character into the first paragraph. It lets the reader know who they are dealing with for the rest of the book. It gives them an initial visual that will shape their perceptions along the journey.
First chapters take time to get right. Don't be afraid of spending the extra time on this part of the book. We need to get it right for our readers. So, no backstory or flashbacks (and please no dream scene straight away), introduce your main character in a sitaution of conflict and make sure we hook the reader right from the start.