gini nih "Formal and informal language"

How coarse or inappropriate to use these words “wanna, gonna, gotta, etc” in formal language? Should I avoid them in my formal and informal language? I read in a book written by a Russian-American woman that it was not good to use them even in informal language. Is it true? I like to use these words instead of normal “want to”, "going to", but I never paid attention to the fact that I might sound grossly. Also I like to insert different expressions in my speech, but I don't know how appropriate to use them in a talk with a person (native speaker) who I don't know very well or meet for the first time. Of course they understand that English is a foreign language for me (but there were some native speakers who laughed at my wrong expressions and I was just like this smiley ), so I don't want to feel shame again because of incorrect use of some word and expressions. Can you advise me how to improve this side of my English?