You'll have to eventually study outside duolingo if you are serious about learning a language.

I would recommend starting with the duolingo course, completing it, then keeping your tree gold while exploring other learning material. Start with children's books and work your way up. There are also a lot of language exchange chat sites that are good to help you get more comfortable speaking a new language, but you will need a partner to practice with at some point.

Also I've found ReadLang (which I found on HN) to be a very helpful tool for faster learning, and I upload the majority of my ebooks into it.

I hope this helps.