The first sentence of your novel is important because it needs to hook the reader. That doesn’t mean your novel’s beginning needs to go down in history; not everyone can be J.R.R. Tolkien with “In a hole in the ground there lived a hobbit,” after all.
One of the most important elements of the first sentence is that it needs to inspire the reader to ask a question. Otherwise, why would they continue? Take a look at the following examples from some of the books on my shelf:
“In the land of Ingary, where such things as seven-league boots and cloaks of invisibility really exist, it is quite a misfortune to be born the eldest of three.”
—Howl’s Moving Castle by Diana Wynne Jones
“The rabbit had been run over minutes before.”
—Sabriel by Garth Nix
“When I wake up, the other side of the bed is cold.”
—The Hunger Games by Suzanne Collins
“I never wanted to go to space.”
—Darkship Thieves by Sarah A. Hoyt
“I have a heart for every year I’ve been alive.”
—To Kill a Kingdom by Alexandra Christo
The questions most of these introductions ask are simple. Why is it a misfortune to be born the eldest of three? What ran the rabbit over? Who left the bed? Why would the narrator go to space if she didn’t want to?
The last example, from To Kill a Kingdom, is a bit more shocking: what kind of person has a collection of hearts? Are they literal hearts? Why does she have them? Who are they from?
Even if you answer the question(s) a paragraph later, that’s okay, because by then, new questions should come up.
It’s also important not to shock the reader with an opening sentence, only to disappoint them because the answer to the inherent question is mundane. For example, if the hearts in To Kill a Kingdom had turned out to be fake or a metaphor, I would have felt manipulated—like the author had began the book that way just to get a reaction out of me. However, Christo’s book is literally about a prince-killing, heart-stealing siren, so the beginning is perfect.
When you begin your novel, consider what question you want your readers asking, and go from there.
Go forth and write!