You can do both the same way. If you're using the visual editor, simply add the link with the link toolbar button. If you're not, and want to know how to create a link with HTML, here's how:
<a href="http://www.yourwebsitehere.com/yourpagehere.html">This is the link text</a>
The <a> tag creates the link with the href being the page you're linking to.