You can look this up, but it's simple. You'll need to insert both the page number field and a field called NumPages, which will show the total number of pages in your document. Once you're in your footer (double-click in it), clink on the Header & Footer Tools tab that appears above the ribbon. Insert page number is there. After the inserted page number type a space and then the word "of" and another space. Then Insert/Quick Parts/Field and scroll down to the NumPages name in the left-hand box of the dialogue; select it and click OK.