While you're writing it, format it however you want. You're the one who's going to be staring at it until your eyes turn crispy.
When the time comes to submit it to a literary agent or publisher, format it how they want it. If they don't give any specific instructions, you won't go far wrong with the following:
- Paper size A4 or nearest equivalent in your country.
- Print on one side of the paper only (if it's a printed submission rather than an electronic one)
- White paper, black ink.
- Generous margins all round (about an inch)
- Font is 12-point Courier or Times New Roman, or something that looks close enough to pass a casual inspection
- Double-spaced.
- Each chapter starts on a new page.
- Header is your surname, a one-word version of your title, and the page number. Eg "Dickens / Expectations / 75"
- No footer, because there's nothing for it to contain.
That's if you're hoping to get published by a commercial publisher (the sort that gets your book into bookshops and does some marketing for you). If you go for self-publishing or print-on-demand, you'll have to make it look the way you want. The company will give you a template to fit the book into.
Learn how to use paragraph and page styles. They'll save you a huge amount of time when you have to reformat the book.