There's a popular phrase in English: practice makes perfect. If you want to learn to be able to understand English well, you should practice listening. Try listening to things that interest you to keep you motivated. Watch films or tv shows in English with subtitles if you need them. Consider getting a language exchange partner so they can simulate a real conversation with you and you can practice speaking at the same time. Find a native English speaker to correct you when you make mistakes and eventually, you will improve.

I agree with Marta. Go out or get in and interact with the language. Meet different kinds of people who speak English in all kinds of ways. Watch a movie, a show or simply a cartoon! The key is to enjoy it even if you don't understand a darn thing! Good luck!