As a writer and specializing in linguistics-second language acquisition, this probably comes at top among all kinds of questions that I get at work from those non-English speakers. I may not be a professional in teaching English, Spanish, or Japanese, but I do have some thoughts to share, which hopefully would help you, too.
My first advice is always about the attitude. NEVER EVER take a language you want to learn as a subject in school! Learning systematically is good, but when it comes to language acquisition, I am afraid it doesn’t help much. Language is something alive, and it varies with the time, the culture, and those who use it. Don’t expect to learn a language well when you think or use it only in the classroom.
Second, start from something you like. If you like fashion, read fashion magazines. If you like cooking, get a nice recipe book with short articles telling where some cuisine comes from or interesting stories from some chef in their kitchen life. If you like watching movies, go dig more from their official websites or join the fan forums. A second language is not acquired like your first language, so you need to get yourself “there” to get started and then expand it.
My last piece of advice is to form a habit of using the language constantly. Mastering a language is like learning how to use a tool well. When you use it/learn to use it everyday, you basically improve not only by making mistakes but also know how to actually use it, well, like how you use your mother tongue.
Well. Happy language learning to everyone!