Question Regarding Procedural Function (store procedure)

Hi, first of all thanks for taking the time to read my message. I'm new to Sql and is very confuse of how to write a procedural function. I want to write a function that check if the data is aready in the table then it should not add to the table instead of linking it to the table. This is what I mean, I have a movies table and a stars table and a stars_in_movies (this table keep track of the stars in what movies). So I'm suppose to write a store procedure that add the movie to the database, if there is a star in the new movies then is already exist in the stars table then it should link that star to the new movie.

thanks in advance

-brian