This is sometimes the most difficult part of a writer's project—the beginning. Where to start? If you were Charles Dickens, you might start with an overview of the setting, then give a character sketch for each primary character. Then after the backstory, the current story would start.
But contemporary fiction isn't like that. Readers want to get right to the story. They want to be dropped into the action as its happening. So where is it best to start your story? Select a scene that will draw the reader in immediately. What action happened that will change the hero/heroine's life forever?
Most often, this is well into your first chapter. Writers often make the mistake of telling all the backstory first, thinking that the reader won't be able to understand what is going on if they don't give them the character's background. But this isn't necessary. You can always add the backstory slowly as flashbacks or through conversations.
If you still aren't sure where to start, try this exercise. Pick up some of the books on your "Keeper" shelf. Read the first lines/paragraphs. What drew you in that made you want to read more?
One of my favorite beginnings is from Ken Follett's Code to Zero. "He woke up scared. Worse than that, he was terrified." In the first few paragraphs, you learn that the protagonist is dressed like a bum, and doesn't recognize himself in the mirror.
Why was he terrified? He can't even remember who he was? Something must have happened to him to drop him into his situation. Did I mention he woke up on the floor of a public bathroom? Just think of all the questions your readers will have just from reading those first few paragraphs.
Immerse your reader immediately. Make them want to find out more. THAT is where you should begin.
What are some of your favorite first lines?