Page Size and Margins
The next thing to set is your margins. These will have an impact on the thickness, and therefore cost, of your book, but don't be tempted to set them as narrow as possible. Your readers need somewhere to put their fingers and thumbs when they hold the book open. Traditionally published books tend to have margins in the region of 3/4 to 1 inch. If you're not sure what to use, find some books of a similar trim size to yours and measure their margins. This will give you an idea of how your book will look.
The POD printer you've chosen may also have their own specified minimum, so poke around on the site and find this out before you decide. For example, Lulu has a handy guide to formatting if you want to have your book available for retail, which specifies that margins must be at least half an inch and the top and bottom, and left and right, must match.
Once you know what your margins are set them in the Page Setup options, and remember to select this for the whole document as well.
Next time, choosing a font and setting up styles.