At the top of your new post box are several buttons. Between the text color button and the align left button is the link button. It looks like a chain link sitting on a globe.
When you want to post a link to some site, type in whatever phase you want to use, like "click here" or the website title. Then highlight the phrase and click the link button.
A window will pop up and you enter the URL there.
If you click the edit HTML tab, you can enter the code manually with the proper tags. It would look something like this, <a href="http://somesite">Click Here</a>.