Pick a size you can actually finish
A first novel of 60,000–80,000 words is normal. That's roughly 30 chapters of 2,500 words. Write that number down — a book stops being scary when it's arithmetic.
For teens & adults
Same craft, different life. Here's how to build a novel around school, work, and everything else that eats your day.

A first novel of 60,000–80,000 words is normal. That's roughly 30 chapters of 2,500 words. Write that number down — a book stops being scary when it's arithmetic.
Beginning, the thing that goes wrong, three complications, the worst moment, the ending. Six lines. You can change all of them later, but you'll never be stranded.
Leave [FIX THIS] in brackets and keep moving. Editing while drafting is the main reason people stop at chapter three.
Ask specific questions: where were you bored, where were you confused, who did you care about? "I liked it" is nice and useless.
Story first, then scenes, then sentences, then typos. Trying to do all four at once is how a revision takes three years.