Edit page content
The 'Manage Content' button > Add a link
The "Insert/Edit Link" button has an icon looking like a globe with a couple of chain links. Hovering your mouse over the button will reveal its title, and clicking on it will provide you with another menu you can use to proceed to add a link to your site. Be sure to select or highlight the text and/or images that you want the link to work on before you click this button.
A general rule of thumb would be that you should set the Target to open in a new window IF the link will take the site viewer away from your site. That way, when they click the link, your site will remain open and the new site will pop open a new window or tab. Then your site viewer can return to your site with ease.
More information can be found about setting links using the CKEditor here: Links, Emails and Anchors.