Centering Page Titles

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:

http://codex.wordpress.org/Child_Themes