Headings make text stand out and help people scan your document.
The simplest way to add headings is with heading styles. Using heading styles means you can also quickly build a table of contents, reorganize your document, and reformat its design without having to manually change each heading's text.
Select the text you want to use as a heading.
On the Home tab, move the pointer over different headings in the Styles 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 heading style you want to use.
If you don't see the style that you want, click the More button to expand the gallery.
Styles can do a lot more, including a table of contents. Here's a short video on why to write first and format later using Styles.