You want to study vocabulary that's important to you, stuff you'll actualy use. Once you get past the basics, for which I would also recommend memrise or really any flashcard-like software, find books that you would want to read in your native language in English, or English music you already like, things like this which you would enjoy regardless of the language. You're brain will be much more engaged this way rather than slogging through textbook drudgery.

It depends how you prefer to learn. I use https://www.memrise.com to learn new vocabulary for other languages. Memrise has a lot of different courses for all languages and all levels. You could also watch english shows online - simple ones might be soap operas or children's shows. Depending on your level I might recommend watching 'Behind the News', which is an Australian news show aimed at children (uses vocabulary that a native english speaker of 11 or 12 years would understand). It is available online (http://www.abc.net.au/btn/) and all episodes have a transcript which you could use to look up words. Otherwise you could look up lists of the most common 5,000 or 10,000 english words or you could read english novels and take note of the words you don't know and try to learn them. Or you could just talk to english speakers. It all really depends on what works for you.