@xplr_girl well it's honestly your choice but if you're asking for advice then I'd say start with the character's introduction at first. If two main leads then either both in one chapter or different chapters with different POVs if you're doing dual one. I would suggest you to keep an amazing mind blowing plot (if you don't fine one it's alright we all struggle to find a great plot sometimes) in average keep chapters of around 1000 to 2000 words and chapters total around 35 to 40 or if you want short then somewhere around 25 to 30 but if you prefer longer ones then go for it, but remember that the lengthy chapters shouldn't bore your readers. Other than all these remember to add some things into the book which would bring interest in the readers reading the book to read more to know what's next. It could be anything even cute cringey fluffs bringing the internet for readers to continue reading. Remember it's alright if you make some grammar errors here and there. That's all I guess I would suggest you to look into for writing a book.
Ps. If it's a Loki fanfic...tell me!! I would read it for sure(◠‿◕) ༼ つ ◕‿◕ ༽つ