Those are the days, staring at the white blank screen of your laptop trying not to pull the hair out of your head. Why is it that you've had five cups of coffee, but still cannot form a concrete thought? Well it's simple really, you have started your next novel with the problem being: the first sentence. This should be the most important sentence of your story. In one sentence your reader can decide whether your work is worth their time.
What the first sentence of your story should do is question the reader. That is how you are going to grab their interest. For example in my book, The Demonic Eyes, the first sentence reads: "Blood was the first thing I saw when I regained self-control." This sentence makes the reader think: What is happening? Who is hurt? Did he [the main character] harm someone? When the reader has these questions they need to find the answers to feel closure.
Aside from having the reader develop a curiosity, it should give them an idea what the story is about. Note that though it should provide the reader with information it should not reveal the whole truth. For example, going back to our example sentence, the reader knows (though not for certain) that the story could be a mystery, horror, dark fantasy, or the like. They know most likely that it is not a romance, or realistic fiction and that it definitely is not a comedy.
The reader also receives the tone or theme of the story. From the example the reader sees that the story is of dark content and that it could involve serious topics. Another great way to show the tone of the story is by having your sentence as the thought of your main character. This introduces your reader to the person they will be reading about for the rest of the book. Here's an example from J.D. Salinger's The Catcher in the Rye, "If you really want to hear about it, the first thing you'll probably want to know is where I was born, and what my lousy childhood was like, and how my parents were occupied and all before they had me, and all that David Copperfield kind of crap, but I don't feel like going into it, if you want to know the truth." Though this sentence isn't concise it does the trick. It shows the reader the main character's personality and his thought process; he thinks a lot. It also shows a mutual disinterest for what many people like to avoid: introductions. It is more beneficial to try this method if your character is relatable or is against mainstream society.
Finally the first sentence should evoke emotion in the reader. In the sentence: "Blood was the first thing I saw when I regained self-control", the reader immediately feels fear. Since emotions are relatable to every human experience if your sentence lacks it, the reader can find it disinteresting. One of my favorite first sentences is from Gillian Flynn's, Gone Girl. "When I think of my wife, I think of her head." This sentence makes me puzzled and curious because it is such an odd sentence to begin a story. Curiosity: that is the key emotion. As I have said previously, you must make your reader curious to entice them. Advertising does this all the time. Think of your first sentence like an advertisement. Draw them in before giving more information (aka the rest of the story).
Though it sounds simplistic it might not always be. As the expression goes: "First impressions always stick" this is your first impression to the reader of your book. My suggestion would be that your first sentence does not need to be perfect at first. Write your story then come back to it during the editing process. By then you will know your characters perfectly and you will know everything scandalous in their life. Scandal attracts people. Write a sentence that is bold and that stands out from the rest. Look at the books that are popular and books in your genre. How do they approach the first sentence? How does it make you feel? Do you feel like continuing the story?
Ultimately what you need to remember is to draw the readers curiosity, make them question what is happening, let them get a taste of what the story is about, and evoke their emotion. Make sure to look at other examples and remember it is okay to write a bogus sentence now then fix it later.