Usually one might expect an article by an experienced polyglot speaking a dozen of languages when reading such a headline. But in this case, it's just a German teenager who's only fluent in German and English, while also learning Spanish and French.
So if you think you'll encounter the Holy Grail of Language Learning here, move on. This is not the article you're looking for. Instead, I will tell you about my experience and show you what has and what has not worked for me. To begin with, I will tell you about...
In elementary school, I basically didn't learn any English. Truth be told, we had English class for two years, but still, I would not say that it had a lasting effect on me.
So it wasn't until I left the elementary school and attended a Gymnasium that I really started to learn English. Oh, so language learning must be so much better at a high school, right? Not really. Looking back, I think what really helped me learning English is that it basically surrounds you everywhere, especially me. I was already used to listening to English music, but I do not consider music a good source for learning languages. I pushed things further when I started playing video games in English. Honestly, the beginning was hard. Starting to learn a language is always hard. I barely understood anything at the beginning. But this is only natural, isn't it? Do children understand anything when they start learning their native language? No, they don't. They see actions in combination with spoken gibberish. And their brains just start to connect them so eventually they end up understanding what these things mean and they start to speak the language.
It works for children, so why shouldn't it work for you? I am not making the assumption that stuffing rules and vocabulary into your head does not work. I am just trying to say that it does not work if it is your only way of learning. Learning grammar rules should only be a backup until all these decisions start sounding natural to you. I consider learning vocabulary more important and useful, as long as you do not start to translate things word by word. You do not have to learn the translation of a word, but instead the idea. Again, I want to compare this to a child. A child does not even have the option of translating its native language to Babyish. Instead, it picks up the words from its environment by learning what the word wants to express, no matter if it is feeling, hearing or seeing.
So here comes the nearly most important element of learning a language:
In four years of school French, I could barely speak or understand any French. After two years of school Spanish I could not speak to my exchange partner in the language I really wanted to learn. I started thinking. Learning English worked, so why not these two? What was different? Right, I was immersed, into games in English and later movies. I also picked up lots of words after I started learning programming and was reading lots of tech related articles in English, but that really is just another way of learning vocabulary since I still had to look them up. I do not know how, but it works. It is weird at the beginning, but it will eventually start to make sense. All those sentences slowly start to make sense, even if you do not know some of the words, because you can get those out of the context.
But as I said, this is not the most important element of learning a language. What's far more important is...
You cannot get immersed if you do not have any motivation for learning the language. You can probably live years in a country, but as long as you do not want to learn the language, you won't. They same goes for a class room. Motivation and commitment are the key elements for achieving fluency. I know, it sounds so simple and yet, I didn't get it. I was attending the French and Spanish courses thinking that it would eventually just get into my brain, that I do not have to learn the language in my leisure time. But now I know that language classes in school should only be considered as a way to ask questions. Most of what you will learn comes from what you do on your own.
About software for learning languages
These can be quite helpful, but some are not. For example, Rosetta Stone seems to be a great choice for language immersion at first glance, but to me, it's not. I think the issue is that this is creating a fake environment for someone learning the language. Sure, games and movies do these, too, but their audience are fluent speakers, not people learning the language. In this case, jumping in at the deep end seems to be working better. So far, I can only recommend using spaced repitition tools for learning vocabulary (for example, Anki or Memrise). Software cannot create the environment things targetting a fluent speaker can.
As I said, what has worked for me and what I'm currently doing is playing video games and watching movies in the target language. If you can, try moving to a country where the language is spoken, because -- let's be honest -- that's as immersed as you can get. Also try to at least pick up a basic set of vocabulary using spaced repitition software (or another way of learning vocabulary if there is one that works better for you).
That's it. Hopefully I did not disappoint you. This article is not based on any scientific research nor does it try to imply that. This is just based on my personal experience.