I have began learning German recently, I am using YouTube videos and the Duolingo app, but I feel that this is not so effective so I would like to ask you if there are any free materials well structured which help me? I am interested also in making conversations; any recommendation for that?
Even if you like to watch videos, I recommend not to watch explicit those "language learning" videos. They are creating the illusion you are making progress, but in reality you aren't.
For learning the first steps of grammar, reading a beginner's grammar+textbook silently will help you concentrate on the sometimes complicated concepts much better than the overwhelming bells and whistles of a video.
For learning your first vocabulary, putting post-its with the German names of things (with their definite nominative singular article, please) on all the things in your household will help you much more. You will also learn adjectives and verbs that way. Seeing the nouns will make it a game to remember the right actions and supplements for them.
For getting in touch with the language, singing along to German music videos will get you much more experience how to intonate German correctly. (But not Rammstein et. al, please.)
For motivation, watching all-German (no subtitles either!) videos of topics you are interested in will make you want to understand that cursed dark elf language much more than any coursework you aren't really interested in.
Wikipedia in English is a good free start. It has well-structured grammar articles as well as articles about german phonetics, pronunciation, and even dialects. For practicing your language for free I would recommand Facebook and chatrooms. For asking questions, this is one of the best websites you can use. And don't forget Youtube!