Are you going to learn a new language from scratch? I mean, something other than English? If that's the case - exposure.
It's probably how you picked up English anyway.
As funny as it may sound, exposure to the language you are trying to learn is crucial. Once you find out what language you are going to study, you should try to read and listen to the language as much as you can- articles, videos, podcasts, news, etc. It takes some time to get accustomed to the sound of the language and at this stage it's not likely that you would understand a whole lot of what you see or hear but it is very important for the foundation of your future skills. In order to avoid sounding like a robot or like someone who has learned words at random, you should first pay attention to features like rhythm of the language, intonation, phrasing patterns and phonology. At first, you should give yourself some time to develop very basic passive skills, such as listening and reading. The production skills - speaking and writing - come later and take more time to evolve, so don't expect to be able to start speaking too soon.
From then on, it's highly individual and depends mostly on your learning type, the type of memory you have and whether you prefer a structured (grammatical) approach or a situational approach, in which you learn chunks of language that are typically related to a specific situation. Probably the best way is to combine things and try various approaches to see what works best for you.