How much should you plan the story when writing a novel?
Some successful writers say they don't start writing until every step of the plot is clear in their heads.
Others, equally successful, say they start only with a vague idea, or a character, or an image and just make it up as they go along.
On the first novel I wrote I took the "make it up as you go" approach. I loved the freedom and spontaneity of that method but found about halfway through that I'd made a mistake. I introduced a minor character near the start and liked him so much that I developed a subplot for him. Only at the halfway point did I realize that this subplot would not integrate well with the second half of the story. I took it out and had to do quite a lot of rewriting as a result.
Now I use a hybrid method: I write freely for the first quarter of the book, say 20,000 words, and then stop and figure out the rest of the plot in detail. Sometimes that does still require changes in the first quarter of the book but generally it works for me.
That's what you have to do: not what I do, but finding what works for you. However, I believe that the less experience you've had the more you'll benefit from working out the plot before you write. Many of the writers who say they don't plan anything have been writing for a long time and it's possible they have developed a strong intuition about what will work and what won't.
Having an outline of at least the broad strokes of your plot may make you feel more secure that you have enough story, be a useful guide along the way, and save you a lot of rewriting time.
(Want helpful guidance all the way from your idea through to publication? Get a copy of "Your Writing Coach," published by Nicholas Brealey and available from Amazon or your other favorite bookseller.)