Deciding to write a book is the easy part, everything gets difficult from there on, but I thought I'd share some tips with you before you get started on writing your first book.
First, you will need to decide whether you will be writing in present, past, third person omniscient, third person objective, or third person limited omniscient. Research what these mean before starting.
Main character. You don’t have to have his/her personality figured out entirely--you can slowly develop your character's personality as you write--but you will need the character’s looks. Hair color, hair length, face shape, whether they have any distinct facial or bodily features that should be known to the reader, etc.
You will also need to have some sort of idea of what your book will be about. You can't really dive right into writing a book without having some sort of idea of a plot and setting. If you start writing blindly, your reader won't be able to imagine anything you've written in the beginning. You don't have to have an outline of your book, but you should just have a general idea and setting.
Page numbers don’t matter. Word count does. If you might want to try to get your book published when you are done, research what word counts are acceptable for the kind of book you are writing. Example: I write young adult books, so my word count range is usually from 50k to 10k. Adult and children books will be different, as will non-fiction.
Search synonyms to words when needed. I do this constantly when writing a book and it's a big help. You don't want to use the same word, or examples throughout your book. You always want to switch it up a bit so your sentences aren't predictable.
Always read! You should always read more than you write because you can get ideas. No, I don't mean plagiarism. What I mean is you can get inspired, and you can get an idea of sentence structure. There are a lot of different things you can take away from reading a book, more so if you're a writer than just an avid reader.
Research writing. I know, but seriously, you should. There are so many writing communities that you can join! You can learn from other writers and you can ask for help (though I would advice not to give too much information). It's really important, as a writer, to put yourself out there. It can be scary, but you will benefit from this whether you plan on trying to get traditionally published, or go with self-publishing, or if you plan to do neither.
If you're writing your first book, I'm here to tell you that there is no way it's going to be absolutely perfect. Finish it and move on to the next, because the more you write, the more you learn.
Anything I should have added?
Happy writing! <3