The behavior you desire would require the email to be in HTML format which, for countless reasons, is not preferred.
If you absolutely must, then so be it. All you do is the usual mail() function stuff to send the message, just write the body of the email in html code.
The success of this method relies entirely on the mail reader being used by your recipient. Lots of people use readers which do not interpret html, so all they will see is tags and a big mess. Some people disable html even on able clients because html is often used in very poor ways and so you will want to reconsider this.