How to learn foreign language?

Some people can speak more than two languages, trilingual and quad-lingual.
Learning languages needs some skills. Some people can learn other language easily, while other cannot do that.

Today, I watched SFC (Shonan Kamakura Campus)'s lecture online. The guest can use Japanese, English, Tagalog, Spanish, and so.
He told participants that finding the suitable materials is the easiest way to improve their language skills. For example, some Japanese housewives are crazy to Korean dramas, and they try to study Hangul eagerly. In my opinion, it is suitable way to start to study new language.
He also told that like a sport, our language skills improve like a stairstep. We need to store some knowledge and experience for it. When these reach to some level, our ability suddenly become better and we can use the language frequency.

One of my friends is an English teacher in a high school in Japan. He told me that there are 4 skills, reading, writing, listening, and speaking. Each skill is not independent. It is a better way to improve all skills to make positive spiral. For example, when we speak English a lot, it is a good opportunity to read a book to find new words or phrases. Those new words can be useful to speak.

In my opinion, it is necessary to stay in an environment which we have to use it. In this country, we can lead our comfortable lives by only using Japanese. Therefore, we should choose other circumstance not to use native language. We can receive English "shower", in other countries.