To insert links in your posts or pages, just use the Insert / edit link button in the visual editor, when you’re on the editing post page, just like the image below shows.
As you can see, you ca also use the hotkey Alt + Shift + A on your keyboard after selecting the piece of text you want to link.
In order to remove it, use the button next to the one youused before after selecting the linked text and your link will be removed.
Using the text editor or HTML
If you prefer to use the text editor, use the link button displayed in the text editor or simply enter the HTML code for links, e.g.:
<a href="http://example.com">Link text</a>
You can find more information about links / HTML, visit this W3Schools page.