Improving your english vocabulary directly depends on your perseverance. I can tell you about my method, of course, it was not invented by me but using of it I consider I was able to achieve worthy result.
You need a notebook, a ballpen and an enthusiasm.
Every day you need to write down no less ten words in your notebook. I advise you to learn them by heart before sleeping. If your memory allows you to learn more words, let's do it ! Thus, at the end of month you can know no less two hundred words. Experts subdivide "passive vocabulary" and "active vocabulary". The difference in that the first helps you to read and the last one helps you to speak. We can know many words but when time comes we cannot pronounce them because they are placed deeply in our memory and we need time to remind them. Only practice can correct such situation. Of course, language practice with native speaker. But we can replenish our "passive vocabulary" using the avobe method. 

Well , it was a small digression )
Now, speaking on business
I advise you to read books whose themes are very interesting for you. You can read them and write down words which you don't understand. Obligatorily, to read journals and articles. For example, if you are a lawyer you need to read appropriate articles thereby increasing your special (vocational) vocabulary.
And personally from my example. I read short stories written by famous classics like Conan Doyle, O. Henry, Ch.Dickens, O.Wilde and others. Why short? Because they do not load brain heavily unlike long stories do. Nevertheless reading foreign literature is a difficult operation. You need to stock up patience and diligence.

I hope, my narrative will be useful )

 There are  several categories that  will  much improve your vocabulary.

   All of them are far superior to fiction, although most recomendations in reading,  unfortunately,  are recommendations  for novels.

      Science,   Philosophy,  Theology and Law  are  key categories for vocabulary development.

   Also,  I think that the study of Antonyms  is critical for vocabulary development.

That way,  the student learns  words paired with their opposite, and with 'middling" vaues for the terms,   develops a full spectrum of knowledge.

   Here is one example of some useful study material:

http://www.enchantedlearning.com/wordlist/opposites.shtml

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 Hello Denise, how are you? Have you ever noticed we don't use all our vocabulary when we're speaking with someone? Even in our own mother language we use just a little of what we really know.

 You probably don't use difficult words when you're speaking with your mother or some of your friends, right? With difficult words I mean that kind of word that you only see in newspapers and books. When we're speaking with someone, we always use the same phrases. 

  I think you could learn more phrasal verbs and try to imitate how native speakers converse with each other. It works better than to spend your time looking for news words on books and newspapers.

My level is B2 and i'd like to read something that could improve my english vocabulary.. i mean, what i can use in a conversation among both adults and child, wich includes words not so easy but effortless, useful .. in my native language (italian) i usually read romantic stories or adventure one..  

I'm somewhere on B2 level as well and one of my best friends recommended me to read a lot. Therefore I started reading a lot and suprisingly I can understand well. But it depends on book. For example some books from P. Coelho are easy to read with my English skills, but for example The Redbreast written by Nesbo is much more complicated for me.

So, just start read something and if it doesn't suit to you, just pick another book.