If you just want to add text to a header or footer, you can simply type it into the header or footer area. However, you can add other types of information by using the Header and Footer toolbar. For example, you can have Word add page numbers in the following format: Page 1 of 10, Page 2 of 10, and so on. You can even do really cool stuff like add a graphic, or a date and time that will be updated every time the document is opened.
The Header and Footer toolbar is a special kind of toolbar: It is only open when you are working with headers and footers, and it doesn't appear on the Toolbars submenu. You must close the toolbar to switch back to the main document.
Once you have header and footer content in place, you can format it in the same way as you format other text, such as changing the font size or color and centering it on the page.