Where to begin
Learning language is like building muscles
I always tell my students that improving their linguistic skills is like building up muscles: If you do not use them, you lose them. This is because our brain remembers best what we do, then what we see and hear, and then what we read. In other words, if you want to acquire a language, then LIVE IT.
So if you have the chance, move to a country where they speak it and, with the combination of the exposure and a good teacher, you will surely be speaking it in no time. Although attractive, this is unfortunately not an option for most people.
However, it is still possible to create your own ‘language world’ where you are highly exposed to your target language.
Create your "language world"
There are lots of things you can do to create your ‘language world’; and the great news is that most of these are for free! Let’s suppose you want to speak English.
First thing you can do is to set your phone (laptop, tablet, whatever device you use the most) to English; if you already know your phone well, I promise you that this will not make your life any harder and that you will get accustomed to it quickly. Your apps should automatically set themselves to English, but if they don’t, then do it too.
Secondly, follow social media profiles that post about topics of your interest in English (like Facebook pages such as the BBC and Channel4, which post short videos with subtitles); watching their posts will help your brain go back and forth from your mother tongue to English.
Thirdly, get yourself a teacher who guides you through your learning process, provides you with the structures, corrects your mistakes, and gives you the chance to practice and improve your speaking.
It is still possible to create your own ‘language world’ where you are highly exposed to your target language.