First we'll assume you're editing/creating an HTML email. HTML gives us more control than plain text. So make sure you've clicked the HTML tab to show the HTML editor.
Next find the area in your email where you want to add the hyperlink. Most of the time we say something like, "Click Here For Complete Details"
Highlight the words with your mouse that you'd like to make your hyperlink out of, then click the hyperlink icon (looks like a chain link)
Next type or paste in the link of the page you want the person to land on when clicking the hyperlink. It typically is best to begin the link with: http://www...
Don't worry about setting the "Target"
When you're done with your other edits, hit the "save changes" button at the very bottom and test the email. Make sure to actually try clicking the link in the email when it arrives to ensure it brings you to the proper page.