How to make the best use of dulling to learn German, and is it enough to be a fluent speaker?
How to make the best use: My method was doing duolingo lessons+ writing the words in an exercise book. If something in the grammar was not clear, I looked it up on the internet.
But sadly this is not enough to be fluent. You will become good in translating sentences, but speaking is a different matter. You should find someone who speaks german very well, and practice talking with them.
Try to read easy german news too. E.g. on the main page of web.de. And search for easy german tv shows like "Die Sendung mit der Maus": http://www.ardmediathek.de/tv/Die-Sendung-mit-der-Maus/Die-Sendung-mit-der-Maus-vom-05-07-2015/Das-Erste/Video?documentId=29394974bcastId=1458 It's for children but some of the documentary stuff in it is rather interesting and you'll get used to listen to german, so maybe you'll get more comfortable with hearing yourself speaking german. And especially they use "right German" most of the time not this wrong stupid people slang you'll hear in most of the tv shows. I don't know if this page is available from outside of Germany, but there'll probably be some of it on youtube.