To add a link to another page on the same website, select the text or image you would like to hyperlink, then use the "Link to Content" button.
Use the auto-complete field to find the page you want to link to (e.g., start typing "About Us" to find the "About Us" page). Select the appropriate page, then click "Insert link" to insert the internal link to the appropriate page.
This will load up the corresponding node for your internal URL and create a properly-coded "relative" link.
IMPORTANT: Be sure to use the "Link to Content" button as described above to add internal links to your pages. Do not copy and paste your full internal URL into the "Link URL" field. This is especially important if you are building a test site that will eventually go live; if you copy and paste a full test site URL into the "Link URL" field, that URL will not update correctly when the site goes live (the URL will still point to the test site).