I had done about 12 months of German with an actual German teacher before duolingo. The theory was enough the get me started, and I then continued learning a lot with duolingo. But as for getting started from zero, you milage will vary according to how many languages you know.

As for what you're asking, I guess it's really hard for english-speakers. I'm bilingual, and coming from spanish and being fluid in english, it's a lot easier. The main thing is: nouns have a gender, and the article ("the") do as well. Do "die" is feminine, "das" neutral, and "der" masculine.

Verbs also conjugate differently according to gender.