In the WordPress Visual Editor

1 - Select the Visual editor using the Text tab in the Wordpress editor
2 - Type in the text you wish to use as a link.
3 - Select the text and click the Paperclip icon in the edit ribbon. This will open up the Insert / edit link dialogue
4 - Enter the URL of the target page and the link title into the dialogue box
Using HTML Code in the Wordpress Text Editor
This method appears tricky if you don't know HTML but is actually very simple and it is useful to know how to do this manually.
1 - Select the Text editor using the Text tab in the WordPress editor
2 - Start the HTML link tag as follows:
<a href="http://your_link_target_here">
3 - Enter the Link description
This is your link Text
4 - Close the HTML link tag
</a>