Read the explanation:
http://www.ecenglish.com/learnenglish/lessons/much-or-many-0
Do these exercises:
http://www.englisch-hilfen.de/en/exercises/confusing_words/much_many4.htm
http://www.better-english.com/easier/much.htm
http://www.myenglishpages.com/site_php_files/grammar-exercise-many-lot.php