It is very simple to create a centered div. When creating some html code in Joomla it is sometimes necessary to use NO EDITOR. In order to accomplish this, go to you user manager and set your editor to no editor and save. Then go back to your article and edit in code view and save. Do not reopen the article once you have enabled your editor again as it will strip out your code.
Below is the code for centering a div:
<center><div>Whatever you want to center</div></center>