When you login to your site it will open on the Dashboard, to edit a page on your site, click at the left where it says pages.
(indicated by the red Arrow below)
This will bring up a list of pages on your site.
Your site comes preloaded with a number of blank pages ready for you to add too.
Mouse over the page name you would like to edit, and click on edit.
With the edit page (shown below) open, click in the field (were it says text here) type the text you would like on the page, when you are finished click on the blue update button on the right to save and publish your new text. This will now display on your site.
If you are logged into your site while viewing it, you will also be able to access the edit page directly form your site page.