Do you mean, for example, that you want to center “About Us” on the main page?
That title is styled by the class “entry-title” in your stylesheet. If you add “text-align:center;” to entry-title you should be all set. It should look something like this:
.entry-title {
text-align:center;
}
But you shouldn’t edit Twenty Thirteen directly. If you’re going to edit your stylesheet you should set up a child theme. Read about that here:
https://codex.www.ads-software.com/Child_Themes