Hello i always find it difficult to plan books because i never know where to start!
Well that is a common problem that lots of people face.
Of course first you need a good idea! If this is your first book I suggest that you should make it as simple as you can. I am not questioning your ability, but it would be easier to start with something simple, but of course you can add your own twists. If you want to do something more complex then I do suggest you do plan it very carefully.
The first place I think you should decide whether your book is:
A character based story
A plot based story
If is is a character based story then I suggest you start planning the characters first! Because if the story is based on the characters you need to develop their character so your story flows more easily. make sure you think not only of their personality and their looks but their past because then you can either use it as a sub-plot or use it to make a part of their character have a reason behind it.
e.g "Joan was an unhappy person, because her parents died in a car crash." though I hope when you say it you put more description in it!
A plot based story needs you to do a short character plan first just so you know who they are but then go on with the plot, do a short description of what is going to happen, list the events, develop it and then when you've done that go back to the characters, you may have made up a good idea for a character while doing the plot.
Thank you for your question!
Have a good time writing! Good luck!