Sometimes, when you're not sure just how you should start your novel, it can seem like a good idea to start with a dream or a flashback, your character remembering something that happened before the reader came along, or something that never happened at all. There are 3 reasons not to do this.
- The Confusion Factor
In the first five pages of the novel, you should be setting up your reader's expectations of the character and world that they're going to be sharing with you for the next 200 pages. If you then suddenly shout "just kidding!" and change everything that they know or thought the novel was going to be about, you risk at worst losing their interest, and at least confusing them.
- It's been done (and done and done and done)
You want your story to stand out in your reader's mind. You want to grab people's attention and hold it. If you resort to this trick that everyone else has tried, how well are people going to remember your book in the midst of all the others? Be worth remembering. It may take a little more work, but it's worth it!
- If it's a flashback, why didn't you just start there in the first place?
If you absolutely have to start us in the "past" and then jump forward, ask yourself why? Why didn't you just start us there in the first place? For a flashback to work at the very beginning of a novel, the event you're taking us back to must be pretty earth-shattering for your character, but then not matter at all until the current time. The fall of Voldemort and Harry's arrival at the Dursleys' home were both HUGE to the plot of Harry Potter. So J.K. Rowling, instead of having a character remember those moments later, plunked the reader down right there in the moment. She started with a deeply significant event and showed it to her readers, and by doing so, those events had more impact.
Finally, you can choose to do a flashback or a dream at the beginning. If it's right for your book, it's right for your book. But you've gotta wow us with it. Make it a flashback or dream to remember!