Don't know how to begin or what to say ? Let me give you some tips~ ^.^

Culture generally refers to patterns of human activity and the symbolic structures that give such activities significance and importance.       ---by  http://en.wikipedia.org/wiki/Culture

You can begin with the following sentences:
    ☆Hello everybody,I come from *** , a country with ***
    ☆Hi everyone,I'd like to say something about *** in my country.
    ☆Good morning(afternoon,etc),I have just read/watched *** . Let me tell you now.

Culture consists a variety of things,such as
    ☆festivals                                      ☆music
    ☆customs                                     ☆clothes
    ☆diets                                            ☆lifestyles
    ☆religions                                     ☆gestures
    ☆meanings of certain colours  ☆taboos                                     
    ☆special constructions              ☆......    

Enjoy yourself here~
                                                                                                                     Love , Kefan
                                                                                                                     Sep.2nd , 2008