"de" is the common gender, which used to be two seperate genders, feminine and masculine. "het" is neuter. "de" is also used for plural nouns.

It's a bit like German in the sense that the article used is usually arbitrary. You just have to learn the article with the word.

However, there are a few rules that could help you that are listed here:

http://www.duolingo.com/comment/1458348