Do you mean in the articles of Joomla? If so then you can use the text editor..I recommend "fck" editor for Joomla!
Install any text editor and make your text look the way you want it. Or use No Editor and use <h1></h1>, <h2></h2>, <h3></h3> tags for text formatting. And read this great thread by Dan Schulz here http://forums.digitalpoint.com/showthread.php?t=413544 first.
You could install a text editor like JCE, which might be the easiest way (if you type the article title with a H1 in the article itself). Just remember to globally turn off "Show Article Title" under Article Manager - Configuration (or individually on every article you create), otherwise you will get dual article titles. You can find JCE here: http://www.joomlacontenteditor.net/