If you are reffering to turning HTML on in your posts or signature, you cant. Only admin can do this and if I am not mistaken it would be a global setting.
If you are reffering to your web site, then you need to search the web to find out what HTML is cause you aint got a clue at the moment.
If you are reffering to your forum that you are running, try telling us what software it is and then bother the writter of that software with such a shamfull question.
P.S. by the looks of it you are trying to run HTML in your sig... Cant mate sorry. Only BBcode, it has enough functionality and with this forum your are allowed to have a *.php as an image.
For BBcode examples go here ->
http://forums.x10hosting.com/misc.php?do=bbcode#thread