I am looking for a word to let others understand that somebody is a person who does exactly the opposite of what he speaks. Is there any word for this?
That sounds like your average politician to me.
It’s hard to give you a word for this without more context in how you wish to use it as there’s a number of words that would fit your description:
Deceitful, insincere, two-faced, false, duplicitous etc.
A dissembler is a person who makes false appearance. One of Wordnik's sources has this usage note:
So which word would be the better of those two would depend on how much the individuals in question are actively portraying themselves to be someone they are not. In other words, are they merely hiding their true nature? Or actively pretending to be the opposite of how they really are?