When it comes to writing any book, first of all, you need to have a clear vision. You need to know where your characters going, what's gonna happen to them, the end, where the story is gonna end up with. This vision should be formed with visual imagery. This is why I told movies are important. Let's take a plot from a plot generator for an example.

ex -  credits - https://writingexercises.co.uk/plotgenerator.php

A man in his early thirties, who can be quite arrogant. 

A young woman in her late teens, who can be quite anxious.

The story begins at a market.
A town is snowed in at Christmas.
It's a story about loss.
Your character reluctantly becomes involved.

So, let's say the man is a software engineer, the woman is a journalist. They meet at a market, the man had lost his ex-wife due to child birth and he is not thinking of another marriage. The woman needs extra money so she decides to be a nanny and she finds the software engineer's place. She is anxious but loves kids, and the little girl of the software engineer immediately likes the woman. The woman's husband two days before the marriage had died from an accident so she is grieving the loss of her soon-was-to-be-husband and never thinks of another marriage. She becomes the nanny of the software engineer's daughter and because of this child, eventually, both of the people get together by growing to care each other in worry and happiness, they gradually fall in love. Make sure you know the grieving experience of a loved one in real before you write it.

Ex - credits - https://writingexercises.co.uk/plotgenerator.php

A young woman in her late teens, who can be quite reckless. 

A young man in his late teens, who is very adventurous.
The story begins on a moonlit beach.
An elderly person is burgled.
It's a story about an adventure.
Your character approaches the situation extremely carefully.

The guy is with his friends in a moonlit beach, he tries to rekindle with his ex but she refuses and leaves while the girl is with her friends too, just having a fun time in the same beach. An elderly lady is also on the beach and a pick pocketer takes her purse and runs away, the woman runs to catch him, the guy sees her running and follows after her and takes the purse. She admires him and talk a little. The guy is an easy-going, never-permanent, never-serious guy while the girl has Borderline so when she gets angry, she is quite reckless and turns the office into a war zone. Because of this, she gets out from every job she gets selected to. When they go home, they have exchanged their numbers and for next two months, there is no talk. The woman goes on working a little job and the guy has rich parents and he is all too easy. However, one day, on the guy's 27th birthday, he decides to enter to a reality TV show of crazy activities (like bungee jumping, skydiving, scuba diving) the girl, having the reckless soul, she decides to attend too (besides she has cash trouble and the reward for the completer is insane) Within this journey, two of them fall in love and the guy learns to be more serious in life while the girl finds comfort within the guy so it calms down her borderline. They complete each other.

See? you have to have a clear vision like this for your story. I recommend this site for anyone for anyone who lacks plot ideas. Make sure the theme is something you have deeply experienced in real life.