Require to Send a formatted HTML Email via Database Mail in Sql Server 2008 R2
When calling sp_send_dbmail, you can set @body_format='HTML' and place the HTML source code in the @body parameter.

Important: I'm not sure if there will be a plain-text version of your message for recipients that can't or won't read HTML formatted e-mail.

Like Mark says, you should check the documentation for sp_send_dbmail for more information. Also, if you want the challenge, you could use the FOR XML construct in a SELECT statement to generate an XML document where the elements are named and arranged like HTML tags.