I have two previous answers that will help you, Here, on the Three Act Structure and Here, on Getting the first 50 pages or so started.
These are geared to discovery writers, like me, but if you already have a plot to follow, they can help you anyway.
The biggest mistake I see beginners make is they want to jump in too early on the "action" or the "big idea", they think it helps to start with a big fight or the aliens arriving or whatever.
This seldom works. The reason is because in a novel we readers need to care about the characters that are in danger, or being hurt, or that are fighting. Nearly every novel (and most movies) actually begin rather mildly; to introduce characters in their "normal world". This is done to invest the reader in the character(s) so they care about them, before you put them in a blender.
In the three act structure (which about 90% of all popular stories follow), the first 10% to 15% of the book is about the characters before any big conflicts or challenges.
The problem with beginners arises because the most memorable part of the beginning is the "inciting incident", something unusual that happens to the main characters about halfway through the first Act. That is after the introduction to the normal world, but the normal world is critical for the reader to know who this happens to so they care about her.
The first Act is about 25% of the story; in an 80,000 word novel that is 20,000 words. Halfway through is 10,000 words. The standard submission format for a novel (which is how I write mine) is about 250 words per page.
So, 10,000 words is 40 pages. That is a lot of writing about your character(s) normal world, their day to day life, their pre-existing relationships with family, friends, lovers, coworkers, their job (or school or whatever else they do most of their day), and other aspects of their pre-existing life in general. We need a sense of who we are going to follow on this adventure, what she is like, how she responds to problems, who she loves, and why she behaves like she does. Who causes her problems?
Part of this 40 pages is simultaneously introducing us to the setting. You don't have to do too much if this is a modern world without any magic, but you do have to find ways to naturally introduce and let us know about anything unusual; like magic, or aliens, or sci-fi elements, space travel, futuristic equipment, etc. Ditto for fantasy worlds, and fantasy cultures.
There is a lot to do in the first half of the first act, and it needs to be done in an entertaining way (not just a long lecture about how it all works; those are called "info dumps" and are terribly boring -- we show people how things work using scenes with characters doing things; we don't just pull them aside for a lesson about it).
But beginners often want to skip this, because they want to get to the drama. Don't be in a rush. Readers like to read! They don't mind that nothing big happens immediately, they are used to stories starting this way, in a normal world with normal things happening, and characters dealing with normal problems.
Clarification: This doesn't mean nothing happens, you also don't want to start with an infodump and lengthy description. You do want to start some drama and conflict from the first page, have your character(s) doing something while you also introduce their normal world. (details in the second link). This just doesn't have to be your main conflict or problem, that begins around the 1/8th mark (with the inciting incident). I recommend you begin with some every-day emergency. e.g. waking up an hour late, due to a power failure during the night. Or the car doesn't start, or the milk she bought two days ago for her breakfast cereal is already sour. Make her deal with a micro-crisis, so we see how she deals with a setback. It is important to make the reader want to turn from page 1 to page 2 to see what happens next; and an infodump or nothing happening will fail to do that.
Read up on the Three Act Structure, it will only take a few hours, so you can see how nearly all modern stories get structured. You can Google that and get millions of hits, if you want to read more.
The "structure" is about what happens, when it happens in the story, and what the results are; broadly speaking it is what you should be writing about in each section of your story.
The Three Act Structure is not like a rule you have to follow or you get rejected; it is actually the result of studying what is common in how very popular stories have been structured. It is just what we have noticed most well-received stories have in common. Following it won't guarantee you also write a great story -- What it can do is help you avoid writing a bad story with obvious flaws.