Imagine if you were watching a movie or a play for the first time. You've never heard of it before, you've never seen any advertisements about it, and you don't even know what it's going to be about. Now...imagine if you could hear the dialogue and everything that's going on...but the stage or the screen is shrouded in complete darkness for the first fifteen minutes. You can't see anything at all. You just have to sort of listen to the words and use your imagination to figure out what's going on. Well, the problem with that is...once you bring the house lights up, once the movie visuals actually kick in...the people watching may have your vision all wrong. And that can be a huge stumbling block.
Writing is no different. In fact, putting out certain details as early as possible in your story is even more important. Because your readers' imagination is three-fourths of the whole experience, and the last thing you want is for them to spend too much time in the dark before you give them the basics. That's why it's so essential to 'set the stage' before getting into anything that will push your story forward.
When someone clicks on your story, most of them are going into it blind. What will it be about? Who are the characters? When does it take place? For example, imagine if you read a story about a guy who's out in the park with his dog, and they're playing fetch, then he sees a guy that he thinks is absolutely gorgeous, and after a brief conversation they decide to go out to a local café for a drink. Seems simple enough, right? Now...imagine that your story has gone on for a few pages, and you don't start giving some basic details to your reader until they're already sitting down to dinner.
Out of nowhere, you mention that both characters are high school juniors. Wait...what? Oh, maybe your readers were picturing them as being a bit older. Ok, well, they can deal with that. They keep reading, and you mention the dog was this giant Great Dane. Whoah! You might have pictured a dog that was a lot smaller, but...no big problem. Then maybe you mention that it's Winter time and there's snow on the ground, but your readers were imagining it being a nice Summer day. Then you mention that it's 8 PM at night, and the guy has a full beard and glasses and looks older than he really is...oh, and did I mention this story takes place in 1975?
What could have been a very sweet beginning to a story has now baffled and confused your readers, because every time you add some major detail that they weren't told from the get go...they're forced to stumble, erase their previous ideas, and readjust their imagination to fit the details you're giving them. Which can sometimes be frustrating and it can give the story a clumsy start. Always remember, this is the internet. There are WAY too many options out there for you to have a clumsy start. The 'back button' is not your friend.
Whenever I start a story, I try to use descriptions of that particular moment so anyone reading can get a real sense of where they are, who they're looking at, and when it's taking place. Now you don't have to spend paragraph after paragraph spilling every last detail at your reader's feet all at once, but you want to give them a sense that they can see what's going on. Because once your audience has dreamed up a solid idea of what everything looks like...that's the vision they're going to be instantly invested in. If you throw them a curveball after they've already made up their minds that the main character is a modern-day teenager from the suburbs, and you tell them, "No, he's really a gritty, chain-smoking, detective from 30 years in the future"...hehehe, we'll that's going to be a problem. If you're writing about a teenager, maybe have your opening scene start out in a classroom, or next to his locker, or at his best friend's sweet 16 birthday party. Maybe have his mother or father call him down to breakfast. These little cues will put the idea in your reader's minds that, "Ok, we're dealing with a high school kid here. Got it." And then they can continue reading, while emotionally attaching themselves to your vision instead of creating one of their own, and later having the two conflict with one another.
In a future setting, maybe have someone fiddling with some sort of futuristic gadget, or describe some giant metropolis with floating holograms and hovering cars. If it takes place in the old West, maybe mention dirt roads and people riding past a saloon on horseback. Whether you describe a corporate office full adult businessmen, or a couple of kids sword fighting playfully with a couple of sticks in a Medieval castle, or walking through an apocalyptic wasteland...try to find creative ways to let your readers immediately know who, where, and when, before you get too involved with any other part of your story. That way you can be on the same page...pun intended.
Hope this helps. Just remember that the faster and easier it is for your readers to get involved in the world you're trying to build, the more powerful your story will be. Set the stage first and then start building momentum from there. The last thing you want to do is trip them up right out of the gate.