Never, ever start your book with a dream sequence.
The wall exploded inward, showering her with chunks of plasterboard and shattered fragments of glass. A deafening, inhuman screech forced her to clamp her hands over her ears as a massive, green-scaled claw forced its way through the six foot hole in the wall. She backed against the jammed door, knowing that the next breath she took was going to be her last, her eyes squeezed shut to block out the vision of terror as the creature’s grasping talons closed around her…
Jenny woke up and turned off her alarm clock.
“What a terrible dream. Oh well, never mind. Time to go eat some cereal and have a chat with my best friend, Tracey.”
See, now that right there is appalling. The reader thinks they’re getting one thing (which may be awful, but that isn’t the point) but they’re actually getting something infinitely worse – they’re getting let down.
The best thing to do if you want to start your book with a dream sequence is to … NOT DO IT.
If you absolutely have to put a dream sequence somewhere else in your book, always clearly introduce it as such and don’t play a stupid game with your reader, who is absolutely not going to find any entertainment value in being duped by an author who thinks dream sequences are clever or fun. They are neither.