Starting a story can often be the hardest part.
I should also preface that the advice I'm giving is going of what I know. Some things may not apply to you. Which leads me to..:

Every writer has their own ways of writing. For me I just like to get on with it and see where my brain takes me, whereas for other people they come up with a whole elaborate plan before they even write a single paragraph. It all just depends on what works best for you. Try things out if you're not sure. For me, I'm just quite a spontaneous person in general, so I prefer free-writing. The only downside to free-writing (for me) is that sometimes you can end up going down one route and then getting stuck, or accidentally going somewhere you didn't want to go. In that case, you have to backtrack, deleting a lot of stuff til you get to a point where you can start up again and go the better route. Think of it like you're driving down a road, and you go one way thinking it's the right way, but you get to the end and see it's a dead end, so you have to reverse out of that road and go down a different one. That's the downside of free writing. Heck, I'm free writing this right now.

Then there's the planning. Now I never make plans, cause I'm too lazy and impatient for that, but I'll try my best to give advice on panning from what I know.
Firstly, work out a beginning middle and end. Where will the story start, what will happen in the middle, and how will it end. The middle should always be the climax of the story, when the most action happens, per se. The most exciting part, the main bulk of the story that defines the whole point of you writing this story in the first place. And the ending should be a resolution to that climax; how was that climax resolved. How did the characters solve this problem they had to face in the climax.
From there we can go deeper, linking this beginning, middle and end. You need an "inciting incident". Something that triggers the "problem" that leads to the climax of the story. Let's use a basic children's fairytale as an example. Goldilocks and the three bears. The "inciting incident" in this case, was seeing the house in the woods (or however the story goes) that causes her, to enter that house and then steal all the porridge etc. That's an inciting incident. (And you can trust me on this one cause I learnt it in film studies woop)

Then through out the story, there should be an underlying character progression. How the character changes from the start to the end. You should also think about how the climax and resolution would fuel this change. It shouldn't be a random change that's completely irrelevant to the rest of the plot. Let's go back to Goldilocks again (even though I can't seem to remember the story lol). At the beginning, she's greedy, she takes all the things she sees (like the porridge) and she's very fussy, specific, everything has to be just right. By the end she learns not to go into random houses you see in the woods... and not to steal things from said houses. I don't know where I'm going with this lol. Anyway, you can see how the character development should relate to the main plot.

This is a good illustration of the basic story structure, useful for when you're planning. Feel free to play around too! You don't always have to stick to this structure exactly. You can end it short on a cliffhanger, or heck, even tell the whole story backwards! (Although this works better in film than it does in written story) just have fun and play around.

I hope this was helpful just as a little starter chapter, even though it's just the basics. Let me know if there's any advice you want me to write a chapter on and I shall do it! See ya in the next one! Happy writing!!