I grew up in Belgium, so in highschool they taught us four languages, Dutch, French, English and German.
At home we constantly listened to English and American movies and we watched about a movie a day. This is why I think in English, and not in Dutch, although Dutch is my mother tongue. We also had six years of Latin, 5 hours a week and this is still very useful, since many languages have borrowed at least some words from Latin.
At university I studied Russian, Slovene and Bulgarian. Back then I was fanatical about it. I plastered the walls with expressions in foreign languages.
Later I went on to learn Slovak on my own. Because all Slavic languages are somewhat similar (like Spanish, French, Romanian, Portugese and Italian are somewhat similar) I can basically read any Slavic language and if needs be I can sort of express myself in all of them.
Good tricks to learn a language are:
-read one of your favorite books in translation. When I started learning Slovak I read one book again and again until I could finally read it without a dictionary. Comic books are particularly handy in this regard.
-watch your favorite movies and series in the dubbed version. I binge-watched Friends in Slovak. This helped a lot.
-surround yourself with native speakers
-get one of those little pocket dictionaries and read it, underline every word you think is commonly used. I did this waiting for buses, on trains, waiting in line, etc. You only need to know 2,000 words to be sort of fluent.
-find out what’s easy in the language and build from there. Start with the easy things first. For example, the tenses in Slovak are really easy. Much easier than the tenses in English or verb conjugation in French.
-find the roots in the language. For example, ‘raz’ in any Slavic language indicates a movement of parting. Falling apart, divorcing, cut up, to spread out, etc. Once you know a lot of these you will find that you can almost guess what a word might mean, especially when it comes to verbs.
-to improve my German I constantly keep YouTube documentaries about World War II playing in the background. I never get tired of it, because the subject fascinates me and there’s a seemingly endless supply of German documentaries.
Notice that I don’t advise you to take a course. I find that most courses are way too slow, that the teachers usually don’t know what they are doing, and that you can go much faster on your own. I think the only time people need a regular teacher is when they can’t push themselves to do the things mentioned above.
That’s it in a nutshell.
The more languages you know, the faster you can pick up a new language.