Go through different exercises:
http://www.learnenglishfeelgood.com/english-indefinite-articles1.html
http://www.learnenglishfeelgood.com/esol-articles-the-a-an2.html
http://www.learnenglishfeelgood.com/mixed-english-articles1.html
http://www.learnenglishfeelgood.com/mixed-english-articles2.html