Simple. If you are sending your mail by the "mail()" PHP function, the last argument is the headers. If you want that the mail software see the message as a html message follow the sample above:
mail(
'someone@ilovebrasil.com.br',
'A test...',
'Hello. My favorite place in the world is my <H2>Brasil</h2>!!!! <font color=blue>Its wonderfull!!!!</font>',
'Content-type: text/html\n From: Daniel <vanham@ns2004.com.br>');
Look the last argument: "Content-type: text/html". It should work perfect.
[]s
Daniel van Ham Colchete