In my opinion, the fastest way to improve is to use a variety of methods and to practice a little bit every day.
The four core language skills are reading, writing, listening, and speaking. Try to practice one or two of these skills every day.
To practice listening, find a television program you like or try watching TED speeches (http://www.ted.com/) or YouTube vlogs (my favorite is https://www.youtube.com/user/gemllama), or listening to the radio (http://www.fluentu.com/english/blog/learn-english-radio/).
There is so much interesting stuff written in English available to read online. Sign up for some e-mail newsletters in English - for example Avaaz (https://avaaz.org/en/). Change your Facebook interface's language to English and 'like' some English-language Facebook pages (for example https://www.facebook.com/TheOnion or https://www.facebook.com/deathandtaxes/) so that their content will start showing up in your feed.
Lang-8 (http://lang-8.com/) is a great website where you can practice your writing.
italki is a great way to meet speaking partners and to find professional instructors who can coach you in speaking. Language is also about culture, and the more people you meet from English-speaking countries, the more you'll learn about it.
Antimoon is a site with lots of great tips for learning English:
Good luck and have fun!