To become one, you do not have to volunteer first, though this largely depends on the recruiting local department. In addition, you must be no less than 18 years old and a GED or high school diploma holder. You can also get more information on the same from this link http://www.becomeopedia.com/how-to/become-a-firefighter.php.
Different fire departments have different policies and methods of applying. Some require you to volunteer and some don't. Your best bet is to contact your local fire department and see what training classes they offer. I'm sure you will be required to pass a physical examination as well.