In my spare time, I teach English as a Second Language. So, you can actually take my advice as a solid one. (This is me blowing my own trumpet) Anyway, for a start, one must determine which level they are at. For beginners and those with little knowledge of English, I suggest that the best way to start is to get a good teacher. A structured way of learning is the best for building a solid foundation in any language. From there, so many other things can be introduced. Movies (first with, then without subtitles), building new friendships with native speakers, ESL games online, etc. All these can come in very handy.