The Genki I textbook goes over the difference between nan (なん) and nani (なに) , which is something I always seem to have problems remembering. It is actually very easy.
Nani (なに) comes before particles and nan (なん) comes before counters and everything else.
There is an exception though, you can use nani (なに) before the counter jin (じん・人) for people.
Resources:
Genki I textbook (http://genki.japantimes.co.jp/about_en)
Learning Japanese for Life 