The simplest way to add headings in your document is to apply heading styles.

Type the text of your heading and select it. Click Home and move the pointer over different headings in theStyles gallery. Notice as you pause over each style, your text will change so you can see how it will look in your document. Click the one that you want.

If you don’t see the style that you want, click the More button  to expand the gallery.

A heading makes text stand out and helps your reader find the information they need. There are several advantages of using heading styles: quickly building a table of contents, re-organize your document, and reformat the design without manually changing the text.

Via Microsoft