The IIBA (http://www.theiiba.org/) BABOK defines a Business Analyst as follows: "A business analyst is any person who performs business analysis activities, no matter what their job title or organizational role may be. Business analysis practitioners include not only people with the job title of business analyst, but may also include business systems analysts, systems analysts, requirements engineers, process analysts, product managers, product owners, enterprise analysts, business architects, management consultants, or any other person who performs the tasks described in the BABOK® Guide, including those who also perform related disciplines such as project management, software development, quality assurance, and interaction design."

You can't gain the CBAP certification by just passing an exam.  You need five years of BA experience over the last ten years. The full requirements are defined here

Getting this certification is not a trivial matter and only arond 1000 people have gained it to date.  

Your first step should be to join the IIBA.

More details on passing the exam are in my blog at http://cbap.posterous.com/