Take out one of your WIPs and look at the first page.
What do you see?
Is it a heavy description of the setting? Is it a flashback or a wordy telling of the back-story? Is it an introduction to your protagonist showing him doing everyday, mundane things?
I’d argue that you shouldn’t do any of these things.
A long-winded description of the setting might be fun to write for the fantasy writer who loves world-building, but presenting too many details to the reader right off the bat will just lose them. All of the details in the world won’t make your setting interesting to a reader who has not yet developed a personal attachment to it. Do all the world-building you want, just don’t throw it all into the opening chapter.
As for why you should never open with a flashback or back-story, ask yourself this: Why do you feel the need to open this way? For the most part, you should be writing in real time. If there are events in your protagonist’s past that need revealing, by all means reveal them. Just not at the beginning. Insert the brief flashbacks (BRIEF, I say! I’m looking at you Stephen King!) only as needed, an only if they directly affect your plot.
The same can be said about back-story. Don’t info-dump a bunch of setup to your story. Just get to the plot and insert the details of the back-story only when necessary. Always remember to keep the back-story in the back of the story.
Finally, if you open by introducing the protagonist (which you should) don’t show him getting out of bed. Or brushing his teeth. Or eating breakfast. Or heading to work. Or performing any of the other mundane actions we all perform every day. It’s boring to the reader and entirely unnecessary.
So where do you begin?
I’ve said this in previous posts, but it bears repeating. Your story begins with an event that changes “normal life” for all involved. This could be something as small as a teenager seeing her boyfriend kissing another girl, or a baseball player looking up and seeing a saucer the size of the stadium hovering over the city.
Go back to your WIP and find that change. Highlight it. That is where your story begins. Now take everything before it and delete it, or put it aside try to insert it later.
You’ve only got one change to pull those readers in, and keep them there. So don’t mess around; start your story where it matters most.