For example, many students have learned to say "De rien" ("It's nothing, you're welcome" ) as an answer to "merci". De rien is indeed very used in France, but more so in the working class environment. French people who fancy themselves as upper class would favor "Je vous en prie" (pronounced shvoo zan pree), and "Je t'en prie" (shtan pree). You will find more expressions of politeness with their proprer pronunciation and cultural notes in my audio lesson French greetings and politeness.
Common mistakes among the French themselves include "la copine à ma soeur," which should be "la copine DE ma soeur." If you speak French like that, it will fly in a café, but not at the Ritz.
So, get a feel for your surroundings, and talk freely in a relaxed setting, but watch the vocabulary you use in a more formal setting, and of course, use "vous."
