"How can I improve my English?" or "How can I take my English to a higher level?" are correct questions. I agree with Manuela - checking your level and getting some guidance will help you a lot. Don't assume that "understandable" means "correct". As well as adding to your vocabulary and grammar, you should find out what kind of mistakes you typically make, and fix them as well.

Either you can work with a teacher that creates a study plan for you or you can buy a few good books and study by yourself. I would advise you to take a test and check your current level, so that you know what kind of books/materials you need. Obviously don't forget to practice your spoken English, too.

If you can access the BBC world service, you may find it beneficial.

You can learn listening to musics, podcasts, watching comedy TV series, vlogs and by books or teachers. I think the important thing is to practice everyday, I listen to a podcast everyday.

I just have one suggestion "Lean More"