You can go the long route, through the File menu to display Word's Page Setup dialog box, or you can double-click!
Word's Page Layout dialog box lets you define margins, layout, and so on. You may use it a lot -- I know I do. To access these options, you probably choose Page Setup from the File menu. It's a short path, but there's an even a quicker way: Double-click the horizontal or vertical ruler.
You can double-click the vertical ruler anywhere to display the Page Setup dialog box. However, double-clicking on the actual ruler display in the horizontal ruler will display the Tabs dialog box. To display the Page Setup dialog box via the horizontal ruler, double-click the gray edge on either side.