ต้องมีอะไรถึงจะเรียนภาษาได้
If it's English, most Thais have already had some background of it since it is the compulsory foreign language in Thailand. Many people start from kindergarten, many from the fifth grade. It continues to be required until people are in college. Why do we not learn it or speak it fluently like people from other countries? When English is a required course throughout our education, we only study it in class, but we have no chance or motivation to use it in our everyday life. For me, it started when I was a teenager. I just realized I loved it, and so I was motivated to be good at English.
So for people with some background like formal education of the English grammar and some basic vocabulary, the only two more things needed are interest and practice.
By interest, I mean you need to begin to notice things in English and want to try communicating in English. By practice, I mean you need to read more. But it's not just grabbing any text in English--you won't really learn much, if at all, if the thing you're reading is not what is interesting to you. Get books or magazines that talk about what you like, make sure they're not too difficult to understand (something that you won't need to consult the dictionary all the time), and just enjoy reading! Without knowing it, you'll soon learn a lot of new words, and the grammar you've studied at school will become alive and make much more sense to you.
If it's a totally new language, one that you have zero background, the most important thing you need is some kind of motivation. It could be program requirement if you're studying for a degree, it could be personal interest, anything. But if there is no good reason for you to learn it at all, why bother?!!
Then once you've started on that new language, vocabulary is one of the things that will help you get better at it. Grammar is important, but just like English, knowing the grammar of a language without any words to put in those grammatical structures is not going to get you anywhere. Then what you need next, also just like learning English, is practice. So practice, practice, practice, and don't give up until you're good at it!
